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I live in Colorado, just outside of Denver.
We have 3% less oxygen here (compared to sea level 18% versus 21%) and almost all our gas is E10 (probably 5-10%)
I beleieve the stoich for E10 is 14.08 (asuming it is actually 10% ethanol.)
The air density in Denver (5280 ft) is about 82% of that at sea level (0.0627 vs. 0.0764 lb/cubic ft at 70 deg F)
How does 3% less oxygen affect a 14.7 and 14.08 stoich?
not much. im in reno at 4250ft and i go to tahoe at 6224ft. i use the same settings from 0 to 6K. i am tuned to remove fuel for higher elevation so if i go to 0ft then my trims start adding back by going positive. It's not too big of a deal. If your concerned, tune in Lambda instead of AFR. Lambda 1 is always stoich no matter what the fuel is.
It is not JUST the oxygen that is absent at altitude....its everything else in the air as well. So stoich does not change...