BITD, the recommendation was 9"-10" less than (cycling) inseam with a 1"-2" clearance. I'm not sure why that's changed. My inseam is a little under 31". I'm between 5'6" and 5'7". My bike it has a 21"/54 cm seat tube and a 22"/56cm top tube. I can ride a 58 cm bike, but when I did, I came down on the top tube once in an emergency. 58 cm is too big.
I can ride 50 cm, too, but I'm too cramped. In fact, I feel pretty cramped on 21" ST/21" TT bikes.
I started riding my bike in 1982. I pretty quickly settled on a 12 cm stem. A couple of years ago, I ran my measurements through Competitive Cyclist's calculator, which came out with French Fit recommendations of 22" ST, 22" TT, 12 cm stem.