I've calculated the ratings-to-sales ratio on products that do have official sales numbers, and used that to estimate it for the ones that don't. I used the median ratio as a guide.

It's hard to tell how accurate the estimates are until some Gumroad seller confirm their numbers to me, but I think with the data I have this is a good guideline.

One surprising thing I found is that the median ratio is the same across all categories, I thought it would vary much more between different niches.