I probably screwed up the math. For a 3.5 gallon batch (4.6 in boil) of Irish Red I used 8.3 lbs. of grain. After mash and squeeze the wet grain weighed 9.4 lbs. Of course, the dry weight of the grain after sugar extraction would be less than 8.3 lbs. Maybe that's the mistake?
Would need to know the total water volume used between strike and sparging.
Weighting the grain bag after squeezing doesn't work very well, as it'll vary with the gravity as well as the absorption rate, it's a formula I haven't seen derived or used before and while I'm sure it's not all that difficult to derive, I don't really care to.
Apparent grain absorption rate ~= (Volume_Used - Preboil_Volume + Mashtun_deadspace) / Lb_of_grain