I might tape a couple plumb boards to the wall top and bottom meas up whatever the height of the bottom of the parallel panel is on the same side of both, (top/top or bottom/bottom sides of the plumb board) and then snap a line across both. That gives you the cut on the top and bottom of the stiles, (vertical trim) and the rails, (trim paralleling stairs) which will always be less than 45deg
I don't have much faith in those angle finders for anything important.
Once the apron is in, (trim at risers and treads) everything else is a matter of duplicating height meas and snapping a line to it, (as long as the plumb bubble on your level is dead on) and your speed or bevel squares aren't mangled.
Work smart not hard!
Never bite the hand that looks dirty