It usually says how far from the floor of the habitat on the box the light comes in. The norm is 12", but each light might be different. Do you have a meter?
It says on the box that it covers up to 50cm / 20" .. can it be too low and give off too much if I had it lower than that?
No, I don't have a UVB meter
Its a T8
10.0 uvb 24" tube.
It is a Hermann, 18 months, indoor rabbit cage currently 80cm x 45cm (will be doubling to 90cm in a month but will get another light the other end then)