I'm installing a new fiberglass whirlpool tub. The directions say: "Ledger board should be the height of the the flange from the floor on which the unit is being installed. For a flanged unit, this height would be the distance from the underside of the top of the unit to the floor".
I don't understand what underside of the top ot the unit to the floor is. The tub has built in feet. Can anyone translate for me?