Tmoni, that is a tough one as I have not done anything like ratings before so I am just brainstorming as I go along so anything I put might be fallable.
I will give it a stab based on a quick think through.
Revenue (17%)
Aim: To find out the firms with the biggest operations based on revenue.Risk (low): Revenue might not be a true reflection on performance, revenue per partner/lawyer might be an alternative but lets keed it simple for now.Methodology: Straiforward. Figures of fee income earned in a year.
No of SAN & Judges associated with firm (Current staff and Alumnis) (15%)
Aim: To find out the top performers in the profession that have been in the firm.Risk (med): Appointment as a judge sometimes can be a political decision rather than a meritious decision.Methodology: to overcome risk, judge appointments should carry less points especially at lower levels.
Hence for every SAN=2 points, Supreme court judge and Attorney General=1.5, Federal Judge=1, State level and Commisioners of Justice=0.5
Baseline highest score to 100 (i.e. 15% of overall) and use for calculating the % rest's scores.
Peer review (10%)
Aim: To find out the firms that other lawyers themselves will want to work in.Risk (high): It is a subjective assessment and can be manipulated if the participants know what it would be used for.Methodology: to overcome risk, it only takes 10% of overall despite being a very good tool for assessment.
Ask a random sample from a few on the top circa 40 law firms their top 5 most admired firm that is not their present employer.
Baseline highest score to 100 (i.e. 10% of overall) and use for calculating the % rest's scores.
Size of practice (i.e. number of employees) (10%)
Aim: To find out the number of people employed[i]
[i]Risk (low): Some firms might employ a few people but be making big dough through high fees. That said, the more you employ, the better the perception of your firms operationsMethodology: Simply get headcount of legal professionals employed
> 50 lawyers = 10 points
> 45 lawyers = 9 points
> 40 lawyers = 8 points
> 35 lawyers = 7 points
> 30 lawyers = 6 points
> 25 lawyers = 5 points
> 20 lawyers = 4 points
> 15 lawyers = 3 points
> 10 lawyers = 2 points
> 5 lawyers = 1 points
< 5 lawyers = 0 points
Remuneration (special focus on salary at intake and after 5-10 years in practice) (15%)
Aim: To find out how well staff are renumerated and that they are not being paid peanuts whilst partners live large.Risk (high): Pay spectrum might be wide for each grade and can't really know what partners earn and it might not be consistent yoy. And I know nothing about Nigerian pay as you will soon see 
Methodology: to overcome risk, use an average pay scale for each grade.
Average pay for new entry > N250K per month = 5
Average pay for new entry > N150K < N250K per month = 4
Average pay for new entry > N100K < N150K per month = 3
Average pay for new entry > N70K < N100K per month = 2
Average pay for new entry > N40K < N70K per month = 1
Average pay for new entry < N40K per month = 0
Average pay after 5 years experience > N600K per month = 5
Average pay after 5 years experience > N450K < N600K per month = 4
Average pay after 5 years experience > N300K < N450K per month = 3
Average pay after 5 years experience > N150K < N300K per month = 2
Average pay after 5 years experience > N100K < N150K per month = 1
Average pay after 5 years experience < N100K per month = 0
Average pay after 10 years experience > N1m per month = 5
Average pay after 10 years experience > N700K < N1m per month = 4
Average pay after 10 years experience > N500K < N700K per month = 3
Average pay after 10 years experience > N300K < N500K per month = 2
Average pay after 10 years experience < N300K per month = 0
Average pay after over 10 years but not partner > N2m per month = 5
Average pay after over 10 years but not partner > N1m < N2m per month = 3
Average pay after over 10 years but not partner > N500K < N1m per month = 1
Average pay after over 10 years but not partner < N500K per month = 0
Average pay of partners > N3m per month = 5
Average pay of partners > N2m < N3m per month = 3
Average pay of partners > N1m < N2m per month = 1
Average pay of partners < N1m per month = 0
Baseline highest score to 100 (i.e. 15% of overall) and use for calculating the % rest's scores.
No of Big Cases/Deals mandated with (Based on Top 20 civil and commercial cases/deals each year) (15%)
Aim: To find out the firms that are chased to handle tough and juicy cases because of their expertise.Risk (low): Deals are based on connections, but connections is part of a firms strength anyway.Methodology: Decide top ten by value of fee income or public awareness of case
Grade 1-20 and award equivalent points to mandated firm.
Baseline highest score to 100 (i.e. 15% of overall) and use for calculating the % rest's scores.
No of partners per employee (10%)
Aim: To find out the firms that promote lawyers to partner rather than only allowing founders to be partners.Risk (med): It does not take into account performance gradings that can lead to lack of promotion and promotions below partner level at newer firms.Methodology: no strong inclination to overcome risk, top firms should employ top performers anyway. Newer firms means less period to access strength anyway and they are likely to have small staff and have advantage with this rating.
Divide the number of legal staff that are non-partners with the number of partners.
Remove a penalty point of 0.2 from score for every partner that is a family member, except the individual:
1) Is a Senior Advocate of Nigeria (shows they are qualified to be partners)
2) Has worked in the Magic Circle law firms (UK), White Shoe law firms (USA), Big Six law firms (Australia) or Seven Sisters law firms (Canada) before joining the family firm and has worked at least 8 years before making partner. (no be beans and pancakes to get employed and work for the foreign firms listed)
Baseline highest score to 100 (i.e. 10% of overall) and use for calculating the % rest's scores.
Employee training programme (10%)
Aim: To find out how the firms takes training of lawyers as a fundamental objective.Risk (high): It is a subjective assessment and can be manipulated if the participants know what it would be used for.Methodology: to overcome risk, it only takes 10% of overall despite being a very good tool for assessment.
Ask a random sample from a few on the top circa 40 law firms how adequate they feel the training offered by their firm is.
Baseline highest score to 100 (i.e. 10% of overall) and use for calculating the % rest's scores.
No of offices (extra marks for having operations outside Nigeria) (5%)
Aim: To find out how expansive the firms operations are and how well strategically located. Risk (low): Some firms might get good business from their poor strategically located offices.Methodology: Extra marks for operations outside Nigeria. Top marks for operations in Lagos, Abuja and/or PH.
5 points for firms with offices abroad and
at least one of the three economic cities (Lagos, Abuja and PH).
4 points for firms with offices in
All of the three economic cities (Lagos, Abuja and PH) and other cities.
3 points for firms with offices in
at least two of the three economic cities (Lagos, Abuja and PH) and other cities.
2 points for firms with offices in Lagos and other cities not Abuja and PH.
1 point for firms offices in cities not including Lagos.
Align scores to % (5% is highest overall).
Website quality (meets minimum threshold) (3%)
Aim: To find out the firms that are professional enough to at least manage their image.Risk (low): None really.Methodology:
3 points for firms with best-in-class website (all sections are complete and look very sharp and neat structure-wise).
2 points for firms with website with complete details AND no malfunctions .
1 point for firms with website with few details and/or constantly malfunctions (including some pages under construction).
0 point for firms without website (including under construction).
Align scores to % (3% is highest overall).