The problem is that you can get several reps on one post. So it's really not a true measure. To get the real number you have to manually go through and count how many unique posts received rep. The number you get this way is still a useful general measure of how much rep you get, but it's really not a measure of what percentage of your posts are repped.
Oh ffs! (I used it! Yay!) - an estimate is good enough, you're talking about doing work... pfft!