How do I use precomputed visibility volume in a distance from the ground?

I noticed that precomputed visibility volume only works when there is something as a floor.

for two different situation in my game there is no floor.

when my character flies and when camera view is top down.

I know for topdown I can increase volume’s height from baselightmass.ini but it will use more run time memory.

how can it work in the air?