BIAB is pretty easy. First thing to consider is the water lost to the grains. I've gone with this chart for a while, using an estimate of about half the amount listed as loss.
Reason being that if you use a bag and can find a way to hang it for a few minutes after the mash you lose a considerable bit less than with a normal mash/sparge. As for a sparge, I'll usually heat the mash to about 168 for 10 minutes at the end to make the sugar more soluble, and then rinse the bag with a gallon or two of warm (not hot) water when it's hanging. Overall efficiency is not too bad, generally in the 65-75% range, occasionally 80+ if I'm lucky.
On a note with the boil volume, starting with a 6 gallon boil for an hour will net about 5.25 post, so you may be looking too high to start with, unless you plan to boil for some while longer.