I feel like having cranks proportional in some way to a person's leg length is a good starting point, but it's part of a larger formula that has yet to be fully mapped out.
Here's my data: my rando bike has a standover height of 32.5", and I can *just* stand over it barefoot, so I figure that gets pretty close to what I'd get with a book and tape measure against the wall. Dividing my preferred crank length of 165mm by that gets me a ratio of 5.08. Perhaps there's something to my femur-to-tibia ratio, or something with my feet that leads me to prefer shorter cranks, I'm not sure.
P.S. Now that you bring up BB height, I wouldn't be surprised if part of the reason for the industry standardizing on 170mm cranks is to fix that aspect of frame manufacture, rather than needing to offer a zillion more frames with varying BB heights. If a bike company were sympathetic to the need for a wider range of crank lengths, perhaps they could scale the BB height with the frame size. Maybe some makers do already...