I've had the same type of issue on some bikes. On a tour I noticed that every once in awhile the chain would hit the head of a screw that attached the fender to the dropout. When the outer link just lined up wrong, it would hit. Solution was to swap out the screw for a button head. Maybe 100 microns of clearance in the worst case, but it never hits now.
How much clearance do you need? Just enough.
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If someone tells you that you have enough bicycles and you don't need any more, stop talking to them. You don't need that kind of negativity in your life.