Pick a lick you like to play often whose difficulty will vary with strap adjustment. I like the infamous Van Halen “Ice Cream Man” solo lick with the big stretch, but just pick somethiing above-average difficulty that you think you’ll play often. Note that depending on the lick, you may not keep the guitar in standard position anyway, so use some common sense about how much any given lick should affect your strap length choice. Think of guys like Slash who hang the guitar low but temporarily brace it against one leg to lift it and tilt the neck vertically for certain licks.
Optionally, simplify the problem by getting a ladder-adjustment style guitar strap, so you have a small number of easily repeatable options to test. If you’re less obsessive, use any strap you like, but figure out an easily repeatable approach for adjusting the length by about an inch at a time.
Start with the longest possible adjustment. Try to play. If you can’t play everything you want, or it’s uncomfortable, shorten by one slot. Repeat until you find a spot that seems comfortable. Make a note of it. Continue shortening slot by slot until you either adjust to shortest possible length, or find a length where it’s not longer easy/comfortable to play. Make a note of it. This should help you narrow down the range of candidate lengths. If you like, try each of this narrowed set of candidates again several times to figure out what you like best.
If you don’t find an obvious favorite as you follow the steps above, adjust to the longest candidate that felt workable. Play it at that length for about a month. If at any point during that month, it feels really wrong, shorten by one slot. At the end of a month, make a note to yourself about what slot you ended the month on, and how it feels. Then shorten by one slot and try that for a month. At the end of the month, test back and forth between that and the previous month’s setting to figure out which is better. A process like this should help you eventually narrow down to 2 or 3 finalists for your favorite slot. For extra credit, after a couple of months you can adjust to maximum length again and try each slot over again to see if your opinions have changed.