Data Report

I don't know about statistics but I would say that there is a good chance of seeing a person besides myself masking at any of the gathering places I visit such as the supermarket or gym. The vast majority of people at these places do not mask. I think this works out to a way higher percentage of people masking than OP estimates, however. I don't think I have ever seen 400 people at a grocery store at one time, so OP's estimated high of .25% masking when one other mask is always visible does not work out for my town of about 16,000. I feel like I would maybe see 25-30 people at a store, so a good chance of seeing another masking person would probably work out to seeing another mask on 3/4 or 2/3 visits. So I'd estimate a masking rate from 2.22% to 3% at the grocery store.