Heighth of a front ( Norvil) fender?

I have a new Norvil "John Player" type fiber glass front fender I want to use. They do not come pre drilled, I assume due to personal choice of tire etc. Is there a good general rule of thumb as to how far it should be off the tire? I'm using a Wm3 with a 100/90/19. Thanks, Glenn.

Glenn,
Rule of "thumb" is pretty accurate, what I prefer is to be able to get a finger over the tyre under the guard. Last time I did this (on a Triumph) was to lay up some cloth over the top of the tyre with the guard pressed down so that the clearance was about right. Then gaffa taped the guard onto the rim so it would not move, then marked up the fork mounting with a felt pen. Remove the guard, measure the bolt spacings and drill through. Note, if this is a fibreglass guard, put some tape on the side that the drill bit comes out as it will prevent splintering tears as the drill bit comes through.

Mick