Is altering temperatures for candy making in high altitudes necessary? The answer is yes. Here is a good guide to help you out.
A good rule of thumb is to reduce the cooking temperature in the candy recipe by 2 degrees every 1,000 feet above sea level.
For a more detailed answer, water boils at 212 degrees F at sea level. But it boils at lower temperatures in a higher elevated area. The best thing to do is to boil water and using a thermometer find out at which temperatures it boils at.
For example, if it boils at 198 degrees F at your house, then subtract that from 212.
212 – 198 = 14
So you would then adjust the temperatures in your candy making recipe by 14 degrees. Have fun candy making!