The PPA calculator isn't bad for postflop use. It adjusts your chances of winning based on folds before you, board, etc.
Like anything else, though, it's just a guide. If you hold KK and the flop comes A, 4,7 it'll show you as a potential winner. But experience tells
you the postflop bet and raise before you means someone hit that ace and you need to fold.
Betting patterns and player tendencies are definitely more important than what the calculator is telling you but sometimes a good read on your opponents coupled with calculator info can help you make a really solid decision.