Originally Posted by C-Rider
brewhouse efficiency is mash only. Boil has nothing to do w/it.
While the above comment is true, the amount of water boiled off or added to the wort will change the OG number drastically.
In you case, When using the linked calculator use 7 gallons to start and end with 6 gallons.
With a repeatable process, grain crush, squeeze, boil rate, etc you should be able to hit your numbers spot on after a few tries.
I sort of do the same process the OP states when I mash with 12 gallons, the grain soaks up about 2 gallons and I sparge the bag with about 3 gallons added back to the kettle, then start the boil with 13 gallons and end up 11.5.... clear as mud ?
But again, my numbers are spot on to what tastybrew says when I use 13 gallons to start and 11.5 ending volume ... and my brewhouse effeciency is 78%