The wainscoting on the stairwell looks more involved. I think a flat rate is the best way to go. It's mostly finish work, paint grade with angled cuts and no coping and not staingrade. So i would add a little to make up for any time discrepancies and snags. Any misalignments you could fill. Also it's repetitive pattern so some templates and jigs could speed up the task? Just my thought never done wainscoting I like simple.