The sight in at 20 and 60 rule is only for HHA sights with the provided set up tape and corresponding sight tapes.

I'll spare the lecture, but the sight tapes were developed using an allgorithum (which I barely understood in college calculus and never learned to spell either).

Suffice to say that you have a set up tape with arbitrary numbers 0-65 on it, evenly spaced (this is supplied three times over with HHAs... along with directions). You sight in at 20 yards... record your number... lets say 16 and walk on back moving a bit at a time until you are at 60 yards. Record your number again... lets say its 56 just to keep the math easy. You subtract the 20# from the 60# (56-16 = 40) THe HHA comes with about 50 tapes numbered like 20-55 or so.... find tape number 40... move the sight slider back down to number 16.... lock it down... peel the set up tape off... line up the 20 setting with the indicator slide... lick it... stick it .... done....

That only applies if you have an HHA and only if you so choose to use the premade sight tapes! Remember that those yardages are only as accurate as your set up... if you guess your ranges and don't make good shots on set up... then you won't have an accurate sight system!

You can also just use a plain white strip and mark off as you sight in walking back.... plenty of folks do this with good results.

Best of luck.