+1

I follow the old constructeur rule of at least 10mm wider on the fender than the tire you're using. This gives adequate side coverage along the tire to protect from spray, even past the length of the front flap. Sure, an SKS P-35 is rated for a 28mm tire... But if you actually use one you're just asking for spray, regardless of mudflaps.

I have a 47mm stainless fender over a 32mm tire (previously used an SKS P-45 fender) and I have no problems with side spray. I made a 4" long, 3" wide extender for the front to eliminate any road spray at all. Attached to the long stainless fenders, this gives me about 2" of clearance from the road, and the entire fender/flap combo is a fairly tight line with the tire.
In the back I used a commercial 10" flap that hangs within 3" of the ground and keeps my riding partners clean of all spray and grime.
Both the front and rear flaps are attached with blind (pop) rivets and aluminum rivet washers. See the attached pix. Fenders are VO 47mm smooth stainless. Flaps are Sammamish Valley Cycles narrow "jelly" flaps. Blind rivets through fender, flap (on interior of fender), and rivet washer for backing.