Altitude Adjustment
Altitude changes baking through one root cause, lower air pressure, and three things follow: water boils lower, gases expand further, and moisture leaves faster. Enter your elevation and what the recipe calls for; this applies the standard corrections for your band and tells you what your thermometer should read for any sugar stage. The reasoning is in Environmental Variables & Troubleshooting.
What the recipe calls for
Sugar work
Why the corrections point the way they do
Lower pressure does three things at once. Water boils lower, roughly 1 °F for every 500 feet, so anything cooked to a temperature endpoint gets there at a lower reading and doughs start losing moisture sooner. Gases expand further against thinner air, so the carbon dioxide from leaveners, the steam, and any folded-in air inflate faster and further, and a batter that over-expands before its crumb sets collapses. And faster moisture loss concentrates the sugar, which weakens structure on top of everything else.
Each correction answers one of those. Less leavening and a hotter oven set the structure before the gas can over-inflate it. Less sugar and more liquid counter the concentration. A little more flour, or a stronger one, adds structure to hold the rest. The percentages shown beside each line are the same corrections expressed for a recipe you work by weight; they are arithmetic on the recipe-unit rules, not a separate set of figures.
Sugar work is the cleanest case, because a candy stage is a sugar concentration and the thermometer is only a proxy for it. Water boils lower at altitude, so the syrup reaches the same concentration at a lower reading, and the sea-level stage temperatures are simply shifted down by the boiling-point drop. The habit that beats any table is to boil plain water on the day, note what your thermometer reads, and subtract the difference from 212 °F from every target.
The full environmental chapter: Environmental Variables & Troubleshooting · Sugar stages and syrup physics: Chocolate & Confectionery · Cake structure and collapse: Cakes & Sponges · Dough temperature at altitude: Desired Dough Temperature Calculator