The aerodrome (r/w) chart should have the threshold crossing hight for the tvasis or papi. If not then they are set up for a three degree slope to a nominal 1000' point approximating a 49' threshold height.
The papi will take you to the same aimpoint regardless of if you are one dot high or low, it is the angle of your approach slope which will change.
Our ops are to disreguard them once you are below 300'agl and fly to an aimpoint.
If you are one dot high on a papi (3whites two reds) you should check your rate of descent is appropriate because you are approaching at greater than three degrees,approx. 3.3 degrees
Now the tvasi with 1 light fly down, 57 to 75 feet at the threshold implies an aimpoint from 1140 to 1500 feet.The landing however occurs ealier because if you are flying a jet or anything larger than say a Kingair your landing gear is some hanging below and behind you. Note that you should see a threshold crossing hight which is where your eyes will be if you had followed the guidance to over the threshold. You are not to know how far behind you until you do a type course suffice that you know whilst your eyes might be flying you to an aimpoint of about 1000 feet your landing gear will touch down sooner (about 650) in most 737s.
However the flare in a jet is the same as a lightie and is always completed by the mark one eyeball on the centreline and a controlled descent to about 1500 to 1800 actual( on a good day)
