Yes, I think that you are right on this. The best figure I have been able to come up with so far is based on some measurements that I have taken from some section drawings that I lifted out of a microfilm copy of 'Fragments of Ancient English Shipwrightry', Matthew Baker, c. 1570
Careful measurement of this and other sections gives an approximation of 30:1, which looks about right. The ratio seems to hold true for any upper deck width, certainly for this period.
I should note that although Matthew Baker is a bit early for the period I am interested in (1637),some portions of the manuscript were written in the early part of the 17th century so it does reflect the evolving practice of the time.
Regards, Frazer