That could be used. But more often, your architect/engineer/drafts-person will have probably designated your existing finished floor ( FF ) as a local benchmark for elevation, and a certain point and edge angle on the existing structure for positional locating.by benchmark do you mean the top of existing foundation? i am adding an addition
Turn it on, as most newer rotary lasers are self leveling. If not, your only required to get the main unit close to level & the unit will level itself from there. As for height, normally you get the top of foundation figured out & go down from there. Crawlspace addition is usually 56" down for modular block (58 1/4" for full 8" H) for bottom of footing. Obviously, a full basement is totally dependent on the # of courses.I need to use a rotary laser to shoot grade for a foundation?