So from the inputs here, 51T is the pinion limit for 90T and 56T is the limit for 87T? I'll probably go for the 90/96T spur, with my existing 47T, I can get down to 4.3, which is probably good enough for 13.5T, and with 51T, I can get down to 3.97, which is marginal for a 17.5? But if I can fit a 56T with the 90T, I can get down to 3.6!