Originally Posted by
squirtdad
I never calculate.... what works well for me for both modern and vintage (friction) setups is small/small sizing
and of course Big/Big combo has to work
Yup. No need for a calculator, and the numbers put into the calculation are subject to operator error in measurement. Small/small gives the right length and if you are cheating by needing more chain wrap than the derailleur can handle then you need to double check with a big/big confirmation and expect the chain to hang slack in the small/small.