The protractor idea will work for those who want to measure degrees of throw instead of inches but you have to make sure that the radius center point of the protractor is EXACTLY on the center of the hinge line or you'll get a false reading. Obviously the inner portion of the protractor needs to be cut away to hold it over the surface being measured, but trying to judge where the center radius point is becomes pretty hard to do.
In most cases a ruler works just fine.