new elevation = old elevation - (new High - old High)/2
If it were only that simple 
This is correct if the deformation extends both up and down with the same length. But take the example I gave in my previous post and the deformation goes 100% down. Same if you have an elevator escape hatch where the hatch door 'falls' down, so again 100% down. With the Velux window it could be 50/50 if the hinge is at the center of the rotating part AND at the center of the rest of the window, but even that is not always the case with this type of window.
It's pretty difficult to determine what percentage extends up and down. After that the calculation to retain the elevation for the non-deformed part is trivial. It should be possible because similar calculations are done for left/right with doors and windows to keep the non-deforming part in place. The same can be used to retain the elevation.
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