You are correct! I definitely made that mistake. I got it flipped around. It’s a proportional tax, and thus regressive to some extent. Making it a progressive tax is what would generate more revenue. However, I’m skeptical that would really be needed.
Yes, that was part of what I was saying; that the ACA greatly reduced the number of uninsured. If I recall correctly the current rate of health insurance was in the 85-86% range. Given the individual mandate part of the ACA (at least, until recently), I assume that number is pretty close to maximum load. If people don’t have insurance through the ACA, why would we expect them to have insurance through a single-payer system? It might tick up a bit, but I assume a lot of that is because people are homeless/transient, expats or incarcerated.