Tricky little buggers huh ?
When I did use cutler rivets, you have to throw precision out the window. The mouth trumpets outwards and the middle bulges. The best remedy was to assemble the whole handle with hidden guide pins, a 3/32 pin just in front and aft of the pin locations sticking up enough to grab both scales during glue up. The easiest way to determine the clearance hole is to pound a rivet together and measure the expanded size then add some more. If you check out old cutlery assembled with cutler rivets, you'll see the holes are big enough to drive a truck through, they relied mostly on compression/ friction fit. Installing the rivet is the last thing I do on a completely finished handle, the heads don't leave much option for shaping unless the scales are left flat.
Rudy