The slide and guess has been very successfully implemented as factory rear sights on many firearms. The Slide and Guess usually has some sort of graduation marks to help the user Guess were it should be moved to for a repeatable setting.

I would Guess that the above linked one has close to .250 of guesstimates available. I have used a very similar Williams Guestable sight on several rifles and pump shot guns with complete satisfaction. they are compact and very rugged.