A bit of geometry might help.
Any cable is most effective when pull an arm at 90 degrees, the same as you would be.
So, you're looking for a length that achieve that when the shoes contact the rim. There's a decent bit of wiggle room here, and overall I tend to prefer cables slightly shorter, but use that as a starting place.
You may want to be a bit brave and buy a spare cable and experiment, starting with it clearly too long for a while, then work down until it's too short, before deciding on your final choice.
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