Tire, Fender, Bottom of Crown Spacing.
How much space do you design in between the top of the tire and bottom of the fender, and between the top of the fender and the bottom of the crown?
Traditionally there should be no less than 1 cm. of clearance between tire and fender. I have used less but have run into frame/fork flex causing the tire to touch the fender. I've also used more but dislike the unsightly gaps.
I prefer to mount the front fender with a large rubber or leather washer and steel fender washer between fork and fender. I usually allow 4 mm for this. So figure total gap from bottom of fork crown to top of tire is about 1.5 cm.