I brew BIAB and most of my recipes call for 10-14 lbs of grain. The thing you are not accounting for is the water loss from grain absorbtion which will be on the order of 1.25 gal. Also, my boil off percentage is generally over 20%. Bottom Line, in a 15gal kettle, I mash in 9 gallons of water to end up with 5.5 to 6 gal. into the first fermenter.
One more thing, for all you BIAB folks out there, go to www.biabrewer.info
You will find an excellent FREE spreadsheet that will be a big help in your biab efforts