Well, I'd give a little bit of padding to the values. Consider that the lowest elevation that you have in the map may not be along the edge where you were mapping. Also, the top may not be the highest point you mapped. What happens if you jump?
Funny Story:
When I was mapping DFC, I mapped all along the edges of the ground floor. Everything there showed as an elevation of 0, so I put my minheight for that map at 0 and the max for the next floor down at 0. When I went to try it out, I discovered that the "worn down" areas between the walls were about at -0.05 or so - just minisculely below. I could jump and it would change maps!
Similarly with the top floor of DFC, I set the heightmin right at the threshold and had to run around on the tops of the walls to be able to see the top floor.
The moral:
Give your heightmin/max some padding and be aware of areas that you haven't mapped.
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