[David]

If your certain popular program is Beersmith I'm here to tell you that you must... must create accurate equipment and mash profiles if you want to get accurate results. Don't trust the profiles that come pre-loaded in the software. Even Brad the developer warns against that. Those are there as a reference or starting point. You must... must take all your measurements, boil off rates, all your losses (actually measured not guessed at) even the material your mash tun is made of and how much it weighs are factors.

It is, and I have. I double checked my equipment and mash profiles when I first started running into issues. Those numbers are correct.

15 pounds of grain at 1.5 water to grain ratio + deadspace + grain absorption from the software gives me 26 quarts of strike water.

Sparge volume is correct.  It's the strike volume that's too high.  You should account for grain absorption but NOT deadspace in the strike calculation.  If you want a ratio of 1.5 qt/lb, I'd strike with just 22.5 qts for your 15 lb grain.  If you strike with 26 qts, you're a gallon high.

The water calculations I used came directly from the software. It has become apparent that I have mistakenly relied way too much on those calculations.

Thanks everyone for your input.