It's really nice to have 10mm or more. With bigger tires, more works better. If you only have 6mm or less, it's likely you will find yourself fussing with the fender pretty often.
When I build frames, it always seems to disappear somehow. Especially with forks. One thing I found was that dual pivot side-pulls actually reduce the clearance available as they are actuated. On my one bike, I just got tired of messing with it and got rid of the front fender. Prior to that, I had switched to a Mafac Racer, which helped a little.