The 5° measurement is worst cast. I have no cherry to break and you're coming across pretty defensive. I'm just pointing out that you can eyeball it and have results that are 99.6% as accurate as a $$$ tool.
So yeah if you can't eyeball 5° (or better) then get a tool that will do it for you.
Don't get too upset that Rusty can't eyeball things he only has one.(a polecat or something like that got the other one when he was young) Besides that he's a curmudgeon.