i think the best answer come from hotelmodemetar at 20 fts you should gently decrease your pl's to flight idle .
In fact the big difference is not between 42 and 72 but between 300 (4 blades) and 500 ( 6 blades)
if you move too fast pl's to flight idle with 4 blades you ll not decelerate too much but with 6 blades youll feel a strong deceleration when your Tq will go under 15%
then if you flare a little bit high with your exact Vref you have 99% risk to perform a nice boom landing with 500 series.
it depends also on how long is your runway
