I'm not sure which table, to be honest. I would assume members? I'm very new to custom profile fields, admittedly I know very little about how they work, and I just used the tool in the Admin CP to create the new field.
5 is full, and -5 would be empty, so 0 is neutral (half full). All users should start with a neutral level, and can either get their Nitrox2 levels lowered or increased.