However, if you do not have time to test your thermometer in boiling water, or if you just want a general idea of how to adjust candy temperature recipes, here is a handy rule of thumb:
Subtract two degrees Fahrenheit from a stated temperature for every 1000 feet you are above sea level.
For instance, if you live at 2000 feet above sea level, your approximate conversion would be four degrees less than the stated candy temperature. So if you were making a recipe that called for the candy to be brought to 240 F, you would only boil it to 236 F.
Another example: if you live at 6500 feet above sea level, your conversion factor would be 13 degrees less. (2 x 6 (thousand feet) + 1 degree for that extra 500 feet.) If your recipe called for 280 F, you would only cook your candy to 267 F. As you can see, the higher the altitude, the more important it is to do this conversion. Even a few degrees can make a huge difference in the successful outcome of the candy.
Remember, this is just an approximation, and for a more accurate conversion formula, you should do the boiling water test on a regular basis.