Originally Posted by
Homebrew01
Why not always do big-big, through the derailleur ? . Then there's never any doubt the chain is long enough, but not too long.
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.