I generally dont look at this way. I look at it on a much more simplified level of trying to play my best every time I play which we all know is impossible but it is a goal. I dont try to quantify results too much for myself but more look at it as did I play my best, did I make the best decisions I could, etc. The thing about freerolls are they have more variance than you think since there will always be at least some if not a lot of players who are basically bingo players. You need to call these players when you have a strong hand but also know your AK suited will lose to their 96 off 35 ish % of the time. Therefore, your results can be really skewed during times you are running well and look terrible in times you are running badly. If you are going to go for a grade scale as you mentioned then I would not use a time frame any shorter than 3 months because of the variance. Secondly, I would get a baseline of where I am at now using your method of am I winning my fair share of the prize pool and then try to improve upon that for the next time period.
The last thing I would want you to think is that "oh I suck cause Im not even winning my fair share at freerolls" which is just not true. Freerolls can be tough not because the competition is as tough but because I feel it is more luck based then your typical buy in game. The bingo players will help you over time of course but they are always a possibility to knock you out on any given hand.
Getting to your grading scale I dont think a C would be just getting your fair share. I am not sure on how results would deem out for myself in this but I would feel like if you are getting "your fair share" in freerolls then it is probably more like A- or B+. I think without having the results in front of me that it is harder to get your fair share than you think. The reason for this is freerolls usually have large fields with the bigger payouts top 5 only so to be way ahead of your fair share you will need to bink these freerolls on a somewhat consistent basis. Thats not easy given their larger variance due to opponents playing styles.
__________________
|