Jags,
Well, basically Jags, ......................ha, ha, don't shoot! I read somewhere (I think it was Peter White Cycles.com) recommending the guard be ten mm larger than the tire for max coverage from the elements, i.e. 5 mm clearance for each side, which would also allow for a larger tire with decent coverage. I don't know if this is best, or not. It just sounded reasonable.
I read your last ride and noticed no pics. Forget your camera? Maybe forgot how to use it? (I have found that any electronics that I own are much smarter than me, and they all know it!)
Tom