I found this article about nitro content and high elevation at http://www.flyrc.com/articles/tune_your_engine_2.shtml .
Tuning For High Altitude Engine Operation
...Run more nitro. Experienced tuners have learned that some of the oxygen can be replaced by increasing the nitromethane content of the fuel; the nitromethane molecule contains oxygen that is released during the combustion process. If you use 10% nitro fuel at home, bring along some 20-30%....