Fair enough... I don't use Promash, I should have said "Beertools" (which I don't use either, but does predict FG), but I do use Beersmith, and it is dead on 95% of the time (the other 5% of the time I just assume I had one too many home brews or my efficiency is crazy high or crazy low). I would be curious to know why your beers are drying out so much... 5 points is significant enough to warrant an investigation. Perhaps your efficiency is low or you are overpitching? You can adjust for efficiency with Beersmith (not sure about the others).
EDIT: One other thought... have you checked your hydrometer's accuracy?
To predict FG, the software has to know what the OG is, and what the actual attenuation achieved by the yeast is.
The OG prediction is a no brainer assuming you have your efficiency worked out, but the attenuation depends on how you mash.
The attenuation figures published by White Labs and Yeast are based on their standard mashes (which may be very different to the way you mash).
I don't think many people would disagree that if you mash at 158F, then you will end up with a higher FG than if you mash at 150F.
I also don't think that most people who understand mashing would disagree that mashing at 150F for 90 minutes instead of 45 minutes will also result in a lower FG
I usually mash for 90 minutes at 150F, and achieve the FG's that I achieve.
Create a recipe in Beersmith that has 10 lb US Pale malt and uses a single infusion full body with no mash out sparge. (158F for 45 minutes) It predicts a FG of 1.015
Then change the mash schedule to a light body with no mash out. (150F for 75 minutes) and It still predicts a FG of 1.015
Frankly, this is B.S, but I'm not sure if B.S is an acronym for BeerSmith or the more commonly accepted definition.
If your mash procedure is similar to those used by the yeast manufacturer and your brewing software, then the FG predictions should be fairly accurate.
If they are different (like in my case), then the FG predictions could be very different.
According to MrMalty, I underpitch. My brewhouse efficiency is 80% (which I don't consider low), and I have calibrated my refractometer which matches my (also calibrated) hydrometer.
-a.