One target for a good fitting is that with the cranks horizontal, the front edge of your knee is over the pedal spindle. It's usually tested with a plumb line. Clearly this criterion affects saddle fore-aft placement. When you are as far back as you can reasonably be, your knee just over the pedal spindle, like this? This "knee over pedal spindle" is often argued over, but to check yourself against this is not a bad idea. It's not a hard and fast rule, however.

Also, if you pedal with the ball of your foot, your leg should not straighten completely (not sure you said it was straightening, but I'll throw this out anyway), there should be a small but significant bend to your leg. If you are straightening your leg completely your saddle is too high.