foxmouth - my explanation is comprehensive so that those of a mathematical persuasion can understand its derivation if they won't accept the simple 60/groundspeed in miles per minute result. Practically I wouldn't necessarily expect students to; all they have to be able to do in flight is to add or subtract 40 and correct things when they’ve flown the SCA time back to track. KISS? Definitely. Timing correction isn't that essential for those poor souls who can't divide by 3.... Why not give it a try - everyone I've taught it to finds it vastly easier than any other system.
It was the AOPA notes I read which had a poor explanation of SCA - and total nonsense regarding timing correction!!