Scorecard logic to measure performance of managers

I need to create a scorecard where I can compare and contrast 3 different managers on one single measures/metric. The problem I'm having is to come up with a defend-able and fair scorecard logic/concept on comparing these 3 managers based on missed deadlines on their respective tasks. Few things I have to mention: These managers don't have equal amount of workload meaning:

- Manager #1 has 100 tasks of which 20 of them missed their deadline
- Manager #2 has 50 tasks of which 10 of them missed their deadline
- Manager #3 has 25 tasks of which 5 of them missed their deadline

At first look, mgr#3 has the least amount of missed deadlines, so he should have the highest rating/score compared to others, but that's obviously not the case as he only had 25 task on his desk compared to mgr#1's 100 tasks. I've tried using weighted average method but couldn't come up w/ something.

I was wondering whether any of you can suggest me a logic that can enable me to rate these three managers based on above-mentioned data. The scorecard can be a 5-star rating, RAG status or a 100 points scale. I am not much concerned about the cosmetic of the scorecard at this point.

Any help/guidance will be greatly appreciated.