It isn’t linear! Battery health drop isn’t linear. By my numbers, it saw a very low - lower than 5% - drop (so, negligible), from when it was new. It definitely, absolutely didn’t see a 37% drop. That’s why I said that battery health isn’t relevant, especially if the device isn’t updated.Absolutely no chance are you getting those kind of numbers - that would put the 6s at 15 hours + screen on time when new which wasn’t the case.
That said, given the battery health you will be being performance limited which will extend the battery life. Can you post the battery report?
Settings > Battery - take a screenshot?
The most I’ve ever gotten is 10 hours of screen-on time, back on iOS 9, with the other 6s I had. Only once, so frankly, that isn’t representative of anything. It consistently was at 8 hours.
Sure, I’ll post a screenshot.