IMO, The only thing that proportional sizing tells you is what the max length of crank is you should consider. Nothing about it shows that you must use that length.
And since your upper leg and lower leg aren't proportional on every person as well as your foot length, then saying one crank size for one leg length is border line silly.
Steve Hogg talks about crank length in detail, and while he somewhat supports proportional sizing, I like the considerations that he describes for different leg and femur lengths as well as other considerations.
https://www.stevehoggbikefitting.com...gth-which-one/
All I can really say is like many other things, you can assume the accepted rules are the truth and never try anything different so you'll never know anything different. Or you can try different things and see what they do for you. I've found that sometimes the rules work for me. Sometimes, as in crank length, the rules don't work for me.