Use the old fashioned protractor?
Consider buying the ruler ... you say you won't use it?
I've said that before, and amazingly discovered once I had it .... it became my best friend!
I got lucky one time, and after my FIRST Shop Hop, won one of those monster 20" squaring rulers (or is it 22"? or 24"?) I always thought they were totally frivolous.
Well, I can't imagine life without one now ... and have "sold" many others on one!