I think 4mmm is probably good; i've seen a range of 3-5 posted somewhere, if not here.  It'll depend on how much debris you think you're likely to pick up on a given course.  I put a strip of chainstay protector along the inside of the cutout to protect the paint.  Every once in awhile it'll make this quick little screech/skid noise when something gets pinched through between the tire and protective strip, then I know i'm close enough.