The way I've always calculated chain length for Shimano 2x systems is either:
Use an online calculator like http://www.machinehead-software.co.uk/bike/chain_length/chainlengthcalc.html
Wrap a chain around the biggest chainring and biggest sprocket to see where they meet. Add 2 pins, and cut the chain there.
I recently bought a bike that uses SRAM Rival 1x gears, and would like to know how to calculate the right chain length for it, primarily so that I can make sure I avoid buying a replacement chain that is too short out of the packet.
The SRAM manual (page 14 of https://www.sram.com/globalassets/document-hierarchy/user-manuals/sram-road/drivetrain/95-7518-007-100-1x-rev-a-road-system-user-manual-eeu.pdf) suggests wrapping the chain around the biggest sprocket and the chainring, and then adding 4 pins. So that's similar to what I've done in the past for Shimano.
Using that method, it looks like I should have use a 55 inch chain. The online chain length calculator also agrees with 55 inch.
However, the current chain on there (came like that from the factory) has an extra 6 pins of length on top of what either of my normal methods think it should be. It is 58 inches.
The bike has ridden fine over the past year, with no noticeable issues. When I come to replace the chain should I replace like for like and go with 58 inches? Or should I trust the SRAM manual and online calculator and go with 55 inches? I'm loathe to "just try 55 inches", as once I've cut the chain it'll be a bit of a pain if I decide it's actually too short and I should go back up to 58.