So marking a straight line on vertical bullnose corners for paint color changes is difficult but arches are even more of a pain. Moving horizontally above your head is harder physically and forget about guessing accurately. Sure you can wing it, try multiple times and still be off center and try to convince yourself it’s not that bad. OR just grab your step stool, slide the BullnoseBoss and a pencil along the arch and you’ve got a beautifully straight and centered line in 30 seconds! Save tons of time and do it right the first time! Remember to place your painters tape next the pencil line not over it so it will be covered up with your paint.