Although the ruler works, you’re right, it's not very accurate. Get yourself a drafting protractor, trim as close to the measuring marks as possible (see pic). This will work better then a ruler, but keep in mind you need to find one that has a standard measurement and not the world renewed metric system that we all seem to forget about, although if you convert over, it shouldn't be ANY problem. If unsure, check initial travel and then convert...Now center the wing incidence with the protractor center and view the throw….adjust accordingly….I’ve never had any problems and of course am open to any other suggestions and or options out there.