It's not nearly as simple as you say. Sure they may have the logs bit then you have other issues like
-not all jobs are the same
-theres utility to calculate
-not all dungeons are the same length
-not all dungeons have the same amount of damage to dish out
-is the grading based on overall averages or on the best of the best
-grading would need to be adjusted with any ability or stat changes
-is communication calculated in this
I could think of some more as well but this is just a few things.