You lose about 3% for every 1000ft. I bbacked this up once with a specific plane i run which I get about 120mph. There were a few at sea level with identical plane engine and setup getting around 145 mph which is a 17% increase for them .
My rule of thumb on the glow engines are to install the larger of the recommended range. Got a 60-90 plane, put a 90 in it. Some of my stock have 90s in .40s. 1.08 in 60s and the whole thing.