In response to to 172
A cautious duck under as you describe it (= closer choice of the aiming point for purists) is surely not a sin. 172 we are in agreement it is part of the duty even, to make sure all the odds are in our favour.
My aeroplane on the G/S has 30 ft RA margin over the THR, leaving
- 10' for me not being accurate
- 10' for a gust or shear
- 10' before the making news.
Geometrically, G/S intersects 291 meters deep. To keep at least 20' screen, aiming for the 200 m point is the shortest sensible option. The extra 90 meters is not a negligible distance, sure. Still, it is a straw-man. Only the touchdown point and speed count. Both are defined by the speed vector at 50' more or less, bit by individual flare technique and largely by correct thrust reduction for the landing.
If we point the A/C steadily to the correct point (not short) between 150-50 feet AGL, cross the THR on the G/S and retard the thrust to idle by 15', the proper touchdown distance is always assured. Mr. Newton and mother Earth will take care of that.
One way to mess things up is to fiddle with thrust through a non-standard flare, having decided at 70' to do something creative. Not aiming short(er) never caused an overrun.