As the air gets thinner you need to reduce your main jet size, so the ratio of air/fuel is about the same.
As you may already know (most of the time-in the winter) higher up in altitude will be "thinner" air. However, that very air (normally*) will be colder, (and denser) therefore, partly counter-affecting the first factor, by around 30%.
That's where this chart comes in. Starting with a reference at 14* f at 6500 ASL, if you need to change temp/altitude, multiply the size by the factor on the left.
* if there is an "inversion" (weird weather) this may not be true.
You can learn the details about jetting, too.
Projects (Karl's) page