Found this:
Wallace Racing - Braking HP Loss at Altitude Calculator
500hp at sea level = 482 at 1200' = 464 at 2400' = 446 at 3600'... linear (18hp/1200')? Really? That doesn't seem right.
I'm pretty sure my numbers are largely due to -me-. I'll get better.
Crazy number of calculators at the site:
Wallace Racing - Automotive Calculators