I disagree as well -- my optimal sleep time is 19 minutes, and I take 1-2 to fall asleep, so I set my alarms for 23 and usually wake up before they go off.

I'm not sure this can be formulated in a way that "works for everybody" though. Aiming for 20 minutes of sleep and doing the picky adjustments of a minute here, a minute there after you're adapted seems the most sensible to me.