A little known fact is that a RD must always have 3T of excess wrap capacity to account for differences in the chainstay length. For any given combination, there is a perfect chainstay length that would require exactly 53, 54 , 55 .... inches of chain. If the chainstay length is any longer, you might only need 1/4 inch of extra chain (1T), but you have to add a whole inch (4T), just to get that little bit of extra length.

To figure out that perfect length, you can use the rigorous chain length formula, found at the Park Tool website.