Generally, 10mm wider fender than actual tire width is what you're looking for. The problem isn't with stationary clearance, but, once you're moving the fenders will shudder/shake and intermittently touch the shoulders of the tread. I've run a 40mm Nokian 240 under a 45mm SKS fender. They do touch, upon bumps, but, nothing major. However, a 28mm Pasella won't work under a 37mm Velo Orange fender. There are too many variables in actual tire widths and diameters to know, for sure, what'll work. Based on my experience with Specialized tires, some of their stuff ends up as much as 5mm wider than the sidewall sizing. Of course, we've crossed that bridge, in the past.