Air bells are pretty normal for one's first roll. We have our personal ways of preventing them. Mine is pretty standard: As soon as the developer is in the tank, I rap the tank quite hard onto a couple of layers of towel on the bathroom floor, four or five times. Then my first one minute of agitation is constant and quite vigorous.

I have never worried about having half a tank if doing only one roll, but one must be aware that inversion in such a case is going to amount to quite vigorous agitation compared to a nearly full tank.