Why use a rule of thumb when the manufacturer of your rear DR has a procedure for determining the proper length of your chain. Shimano and SRAM at least make their instructions available with a little bit of looking.
Usually it's wrap the chain around the big/big combo without going through the RD and then add 1 or 2 links. But not always. Shimano has some different procedures for some of their products and sometimes it makes a difference if the bike is 3x vs 2x.
But assuming the original chain was sized correctly, you just use the same number of links. They don't grow more links with use. But it's also possible that your bike will shift fine with one less link. It'd probably also shift fine with one link more than the previous chain.
Big thing to look for is that your chain doesn't have the RD stretched to it's limits when in the Big/Big combo. Nor do you want the chain to droop and be slack when in the Small/Small combo.
Last edited by Iride01; 06-07-23 at 09:42 AM.
Reason: spelling