You have the dead space listed for your mash tun and not BK, right? I don't have the software in front of me, but I believe there is a dead space option for the BK. If you had 1.5G there, .5G to trub, and .5G to boil off, that is 7.5G needed in the BK right there - for a 60 minute boil.
If you listed it for a longer boil, that is something to account for as well. Is your boil off rate listed correctly? That is an option you can change, the percent expected boil off. You need to figure this out for your system and put that into BeerSmith - and I then adjust that seasonally due to air dryness.
For efficiency - the default is 75%, but if you consistently get 82%, then you can put 82% as your system efficiency, and the software will then calculate your recipes correctly assuming that efficiency.
Brewhouse efficiency is simply - how efficiently are you extracting the fermentable sugars from the mash (that alone is mash efficiency) including the added density during the boil.
I have a RIMS system and get 80-81% pretty consistently, unless I'm doing a heavy gravity batch and still trying to hit 10 Gallons. Then the added grain causes it to drop to 70%. If you keep your system dialed in, that is the goal. Hit your percentage, then you can calculate your recipes better in the software and hit your targets.