In a 172, I've usually got the power all the way off well before the flare, even with a full load -- the airplane is almost kite-like, and unless you're flying an instrument-style 3-degree approach path, you just don't need much (if any) power the last quarter mile or so. In any event, while having some sort of starting point helps, I'm not much on holding "canned" power settings on final. I prefer to set trim to maintain desired speed, and adjust power as needed to set glide path. That way, compensation for differences in weight, wind, etc., is taken care of without great mental gymnastics to determine necessary power.
