I don't know the numbers for the general population, but Fig2 [1] illustrates the difference between the high-carb and low-carb arms between the elite athletes, and Fig3 [2] shows the difference in absolute fax oxidation rates between the groups. If you're interested in finding out more, I'd also recommend doing a literature search for "maximal fat oxidation" (MFO, FATmax) - eg, here's a recent (2018) review w/ Fig1 that shows MFO peaking at 0.72g/min [3] (the FASTER trial cohorts avgd 0.67g/min for the HC arm, and 1.54g/min for the LC arm)

If you're interested in some of the other physiological impacts of keto adaptation, here's a presentation Volek gave a couple years ago that helps contextualize both the FASTER trial and a related TANK trial his team ran, as well as some other interesting related performance research: https://www.youtube.com/watch?v=BeS_dhM8dsY

[1] https://www.sciencedirect.com/science/article/pii/S002604951...

[2] https://www.sciencedirect.com/science/article/pii/S002604951...

[3] https://www.frontiersin.org/articles/10.3389/fphys.2018.0059...

At that effort, close to 0. Maybe 20% for someone with great genetics.