I have 4dB antennas with .5dB loss due to cable/connector so I enter 3.5 on my AP60
I suppose Aruba engineers could put some sort of bound checking in, and maybe they have. I couldn't say.
Just put the gain of the antenna you have into the box.
I suppose I'm still missing your point, since that sounds too simple.
If so, can you make the question more specific? What antennas are you using, and what number are you trying to enter?