Two cuts in any offcut and a hammer tap knock out a notch that hooks around the horn of the window stool — hold the jig there and mark your apron and header lines instead of trusting your eye, which is never as good as you think.
A finish crew's version of scribing base to the floor: tack every piece in the room at a uniform height, run a step scribe along the floor on all of it, then pull it, rip the scribe lines on the table saw, and reinstall glued and nailed — tight to the floor with crisp outside miters.