I’ve discussed my issue determining chain length for use with SRAM Transmission on my bikes in the past.
TLDR; my bikes have rear centers well below what SRAM allows so I can’t use the calculators online or otherwise.
That study was entirely theoretical based on published information that wasn’t entirely understood. Was the SRAM data the result of a calculation or practical testing? It would be nice to know for sure.
I continued to have doubts about how this would work so I continued to think about the problem. While I was mounting the new SRAM 90 system to my RD+ bike, I made another push. I kept imagining that the problem needed CAD or calculations to solve. Silly me. In the end, it worked out to be much more simple than that.
We need to clear the pully knuckle. That’s it. Set up the chain as specified in the instructions. If the chain is too long, it will fold behind the pully knuckle. Reduce the length and repeat until it clears. Simple.