Woohoo, the sixing rings arrived today, excellent service from MM and the postal service. About 10 days from Texas to Australia.
MM was really good with hitting the timeframes that they gave when I ordered. The sizing rings really helped me get the fit nearly dead on for my QK. Take your time with them and ensure that you are getting it right, the better job you do getting the sizing right the better your cage will be when it arrives.
I think i've got most dimensions sorted, i'm just not sure about the cage diameter. Ive had the three smallest rings on and they all seem ok..having said that i'm nut sure of howtight/snug they should be.
Cage width is not as important as length, so being a little looser this way is not so critical. Go with what feels comfortable to you, but not so tight as to possibly impede the flow of urine.
Billus is giving good advice. Restricting flow can be dangerous. Go with what fits the head best, or contact MM about a custom order.