Originally Posted by
davidad
You have got to be kidding! The idea of a formula strikes me as rather much ado about nothing.
Shimano recommends that the rear pulleys be verticle when in the large small combo. I just make sure that I can shift to the large large combo without tearing off the derailuer.
As the responses here have illustrated, there are several valid ways to size a chain assuming the rear derailleur is compatible with the gearing. If it is, any of them will work. If it isn't (as in my case) big-big+1" assures you won't break anything.