You can do more precise elevation math by applying ratios.
If your height is X, on a normalized map you apply a Z scale value that makes that accurate to ±1m
(Default of 100 should be 125m. Test it out. The range is either 256/2 or 512/2 cant remeber wich but trivial to check, just make a default landscape with a black to white elevation texture, plqce a cube. Read its Z).
The position above Z 0 in engine depends on your normalization.
If your map elevation is all above ground you can scale the result between 65535/2 and 65535 to get only positive (above z 0) values.