As long as your conversion efficiency is 100%, it's really easy to calculate what the efficiency will be for a no-sparge. It's just the ratio of the mash runoff to the total strike volume.
For example, if the grist weight is 12.5 lb (1.5 gal absorption) and the pre-boil volume is 6.5 gal, the efficiency will be 6.5/8.0 = 81%.
You do realize you are only calculating a volumetric efficiency of the water utilized, correct?
With a 12.5 lb grist your theoretical gravity points would be ~450. With 100% efficiency that 6.5 gallon batch would have a gravity of 1.069, 80%=1.055, 70%=1.048, etc...with a no-sparge I believe most people with an adequate crush should see their efficiency in the 60-70% range...YMMV