It seems that I am using the rule of 4T more for the guard than the largest ring. Note that these guard rings are relatively heavy. A lighter option would be a ring that meant to work with a plastic guard, if you can find one in your desired size. As to to a dedicated guard for single vs double crankset, it might be the issue of space for the crank. (When necessary you could always trim the ring on your own.) It could be also an issue of mounting bolts, but they mention none in their offer.