Profile gauges are handy in theory, but honestly, I found them more trouble than they're worth for trim work. I tried one when dealing with some old, wavy plaster around my dining room windows. It was okay for getting the general shape, but transferring the outline to the trim was awkward, and it never seemed to fit quite right. I ended up just using a scrap of cardboard to trace the weird spots—quicker and less fiddly. If you already have a profile gauge, maybe give it a shot, but I wouldn’t go out of my way to buy one just for this.
I’ve tried those profile gauges too, and honestly, I had a similar experience. They’re neat in theory but just felt clunky in practice, especially when dealing with weird old plaster. I usually end up using a cereal box or something to make a template. Just trace, cut, and check the fit—way less hassle. Maybe I’m missing some secret trick, but cardboard’s never let me down.