Loving playing with this rule.
With 580 and 730 bars I'm playing with +/-75mm. Current stems are 120 and 90 respectively, so that would mean a 195! on the 580 and a 45 on the 730 for equivalent handling. It would be 10! on an 800 bar.
It seems my old school 580/120 is surprisingly fast. I've never realised over the years of people moving onto shorter stems and wider bars that I already had a comparatively short stem. Feels really sorted and immediate to me.
The 730/90 is very recent on a slacker new bike with an extra 20mm travel, but apparently the steering is slow. Feels fine to me, although I could easily chop at least 30mm off the bar.
I know it's only a guideline, I just found the notion of 195 and 10 stems amusing. Have they ever even made them?