Joe, to "follow" the factory edge angle you can mark up the edge with a magic marker and just keep increasing or decreasing the angle until you are removing the magic marker and not anything else. And LISTEN TO THE SOUND IT MAKES. You can tell a lot just by the sound it makes and the way it feels when "you've got it right". It feels smoother and the sound isn't harsh but a sort of smooth swishing sound? You'll catch on. Freehand isn't really at tough as some make it sound, but it takes some practice to keep the angle constant.

Hope this helps.