Therefore, I propose a slightly modified interface, as shown below.

1 - Elevation underside
This is the height above ground level where the floor starts.
2 - Floor thickness
No explanation needed here. Except the wish that floors can have a thickness of 0 (zero) and be absent.
3 - Elevation topside
This is the sum of 1 and 2, and should be calculated by the program. It serves mainly as information for the user.
4 - Height above topside
This is the height of the level. In practice, this will be the height of most walls on that level.
In my view this is a more straightforward and easier to understand solution than the one currently used.
Hans