I based my 15,000' on using a few different air pressure based altimeters that are all accurate on planes. I calibrated each before taking off and got to altitudes of 10,000-15,000’. You won’t pass out at 15,000’ if you’re sitting in a chair not doing anything. Chacaltaya is above 17,000 feet and I skied there the first day up from sea level. I didn’t feel great, that’s for sure, but I also didn’t pass out. I appreciate the FAA may require 8,000’, but I’ve never had a reading that low. I calculate the effective altitude on probably 1 out of every 2 flights I take.