This is interesting and everyone is different. I will also burn in 15-30 minutes if I don’t wear sunscreen - and my skin pretty much doesn’t tan. I’m either white or burned red.

I don’t live that much farther north. I grew up in central Oklahoma (now in KS). I have lived at high elevation in the Rocky Mountains (7000+ ft) and worked at a summer camp (outside all the time) in the Austin/San Antonio area of Texas, and 15 has worked for me. I have to make sure I put it on 30 minutes before I go outside.
30 gets 4% more protection than 15 and 50 gets 1% more than 30 (based on a basic, quick google search - so who knows how accurate that is, but it sounds about right from previous research).
ETA: I hope this didn’t come across as rude. I believe each of you - our skin is different and responds differently.