Originally Posted by
Sixty Fiver
Big and big plus an inch without the derailleur always works as long as the derailleur matches the setup, and can handle the maximum cog size and total capacity rating of the system.
There are a few circumstances when getting the right chain length is a little harder, wide half step plus granny gearings can push a lot of older derailleurs to their limits but you always set things up so that the bike can be run in the big / big without causing damage to the bike.
It's so rare that we disagree, that I can't pass it up.
While B/B+1 is bulletproof with modern slant parallelogram derailleurs, who's idler cages can rotate enough to clear a straight line along the bottom of the sprockets, it can fail miserably with an older non-slant derailleur. Those lower pulleys cannot swing above a nearly straight lower loop and the RD will be torn off. (wasn't rare BITD)
Older RDs on vintage bikes must have the chain measured big/big,
through the RD cage (a la Homebrew) and pulled up short of tight (one inch or so).
And AGAIN, any big/big method is about the minimum length, but you have the option to go longer.