I stopped because I thought this was enough of an increase for now. The balls of my feet were fine, just a little tingly with sensitivity, which is why I did not want to push it.
Once again, it was interesting to see how how my bare feet automatically adjusted to the hard surface by striding shorter and using a gentle, proper foot strike.
I have to say that I am enjoying it barefoot on the treadmill much more than with shoes! Huh!