I was attempting to install a vinyl new construction window w/nailing flanges in a shed/playhouse. I built the walls first so there not anchored to the flooring yet but I decided to go ahead and install the window with the walls propped up i in my garage. It then dawned on me even if I leveled out the window and fastened it into place then when I go actually set my walls up and level them. then most likely the window isn't going to be level. Am I right in assuming window MUST be installed after walls are set and in place?