PAPI's can be set for different eye heights - a B747 isnt going to see the same as a Citation. They are useful as a guide from someway out but <250' they can become misleading. What they are useful for though is that they are ABEAM the aiming point so provided you keep that aiming point in the same place in your windscreen then you should arrive at the threshold at the correct height (50'AGL) ready to commence your flare.
This is basic stuff - people that chase the PAPI's <300' are setting themselves up for a fall - just keep it stable, with minimal control inputs. A GLANCE in at the V/S should CONFIRM that ROD is on target - don't try to chase or "set" V/S!
Sorry for the thread creep.
A4