Would suggest if you really care that much about the efficiency that you use more sparge water and boil down... does force you into a longer boil, which you might not want. Thicker mash brings its own changes - in that it favours the alpha amylase due to sugar concentration limiting the overall conversion. Basically if you do a thicker mash it's almost as if you mashed at a higher temp *but* just lowering the temp to compensate doesn't work.
I tend to just assume about 60-65% for 20lbs of grain, 65-70% for 15, and 75-80%+ for 10... and throw more grain in to compensate.