I use the general rule of 10mm minimum difference between tire size and fender width; which allows for a 5mm (approx 1/4") clearance all around.

This works well for riding in the lowland PNW where we don't get a lot of solid precipitation. Rain, no matter how heavy it's falling, doesn't pack up inside a fender like snow and slush so a tight clearance is optimal. You want a wider fender with a deep profile so it actually wraps around to the side a bit. This gives maximum spray containment, and when coupled with a mudflap up front, you can eliminate >90% of the road spray. That's not to say you won't still get wet from the rain, but at least rain doesn't contain all the oily grimy bits that the road spray does.
I've got my brevet/commuter/distance bikes set up this way with 32mm tires and 47mm stainless fender on one, and 45mm plastic fenders on another.

On my CX bike, which I ride on muddy trails, I have a crud-catcher on my downtube, and a seatpost mounted splashguard on the rear. All either one does is keep some of the trail **** off of me; at least the stuff that would go right up the middle. Good for mud/snow clearance. Crap for actually staying clean.