I looked at a few of of the Driveline guards around the home and Litepro on-line and the formula for their diameter seems to be D=0.4cm*(No of teeth)+1.1cm. Stacking this against the 22.3cm diameter of the plastic guard that Brompton provides for their 50T ring yields 53T or 3T more than the largest ring. I.e., one should be able to go with just 3T more than the largest ring. Also looking around the home and on-line, some of the alu rings seem well lighter than other. though presumably not as a tough.