Most bicycle fender/rack eyelets are tapped for a 5x.8 metric screw. Likewise most racks are drilled for 5mm screws. If your frame uses 6mm fender eye screws, go ahead and drill the rack to the nearest close size above 6mm.
But double check that your frame is in fact tapped for a 6mm screw because that's fairly rare.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.