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There are different methods but I just use the big ring / largest cog routine as below. Has always worked for me - including the modification for 1x set-ups:
https://www.parktool.com/en-int/blog/repair-help/chain-length-sizing
Adding one link at a time until it is right means you could end up with more joins than necessary which I try to avoid, and also you’d end up with your multiple joins at the same part of the chain.
Having said that 2 joins is fine. I have done this on the Surly (1 quick link and 1 joining rivet, separated by half a dozen links) as standard length chains weren’t long enough for 1x11 + Trucker chainstays.
Nice. Is the foolproof way to just add one link at a time until it works on big chainring/cog and small chainring/cog? It's a triple chainset, BTW.