I think with the 17" GPs, most you will see are 9" and 9.5" and even at that they look lovely and deep. I know that's Duchy's general sizes on the 17s.
Offsets on Banded steels is normally pretty easy to work out on the basis that the face of the wheel doesn't change position, its the bands that are added to the outer face to give the deep looks, and all the banding guys I know of, always band in half inch increments.
It's only the 19" RRs that have the banding both sides of the face because they start off life as a space saver.