Originally Posted by
Retro Grouch
Now I am going to argue.
The beauty of the big/big method is it produces the shortest chain that will safely cover the big/big combination. That's important. If your chain is shorter than that, and you ever accidentally shift into the big/big, you can do serious damage to your bike in less time than it took me to type this sentence.
+1
The big/big +1" produces the MINIMUM chain length. If you're running a derailleur with excess wrap capacity, you can always go longer (as I do), but you can
never go shorter. Over the years I've known too many knowledgeable riders who run derailleurs over capacity, and opted to pinch at the short end. Famous last words for drivetrains and rear hubs are "I never cross chain anyway".
If you run a derailleur over capacity pinch at the long end. The worst that can happen is you shift to small/small and the chain hangs down like a pregnant possum's belly. But nothing will break.