22.25 mm is 7/8th there will be a tiny gap (some wise ass will tell you) but the width isn't the problem its the depth you need the metal to be true flat the end mill needs to be razor sharp and not skip plus you need the mill to line up with the hole you have already drilled for the Atty it's like patting your head whilst rubbing your tummy whilst talking to the wife and turning the telly over slowly, to deep and the Atty will be sunk so the top will scratch too shallow and it will leave a gap
other than that its a piece of piss
don't you just love mod making
edit the pressure on the top of the case will be very great so be careful of that also.... sorry