Help - ceiling thickness

[ranma400]

Hi there.
I'm using Home Designer Suite 2014
I'm afraid I'm new at this, so am stuck on some probably quite obvious things.

I have some floor and ceiling heights for a two storey building - measured from the real thing.

Ground floor at 0
Ceiling at 2080 mm
Floor above 2240 mm
Ceiling of upper storey 4610mm

I am trying to enter these into room specifications, but absolutely cannot.

The settings seem to keep changing themselves so that this combination is impossible.
When I change Ceiling (B) to 2080, Floor Above (A) changes to 2398

I think it might be because the Ceiling structure and floor structure settings cannot be changed, and these may reflect the thickness of the gap between ceiling and floor above.

If so, how do I change these?

Thanks for any help you can give.

Paul