How accurate is PokerGrader?
 

Okay, I have been playing around with PokerGrader for a while now(cool program, by the way). One thing I use it for is when I have a losing or break-even session, I will often upload my HH files to see how my actual profit compares to my potential profit. If my actual profits are way less than my potential profit, I feel good because I know I played winning poker but just had some bad luck.

How accurate is PokerGrader in determining actual profits vs potential profit? Is my reasoning accurate here(losing session, but positive potential profit=good thing)

Thanks.