Using a caulk line to extend a 4' level line to 20' is very open for error. Just a sixteenth off in the 4' will give almost 4" in elevation difference in the 240' of building-if dropped 1/16"x 5 (4') each 20'. May work for 20' as you said, only off 5/16" that way, though I would never suggest that procedure to anyone for any elevations- other than something 8'maximum long with a straight-edge and the 4' level. Using a 4' level alone is accurate to 4'. I think the OP is trying to find the high/low ones to plane them all, too late in the ball game to level everything if he never checked the slab before framing began.... could get the bulk out.