The Holiday season is one of the times of year when Candy Thermometers are often used. I have actually given one as a hostess gift before. I feel it is that random item everyone should have. Also if you are getting one, the digital ones are awesome.
I asked this question to a professional recently and felt so silly when I heard the response. Candy as some may know is tricky because the temperature is so critical 10 degrees off can make all the difference. I had been making a lot of marshmallows recently and noticed that my thermometer had not been rising above 200 degrees and it seemed odd. I put in another thermometer and sure enough it was already at 260 and it only needed to go to 240. In the end it just made a massively sticky mess.
So here is the little trick water boils at 212 degrees. Place the thermometer in a pot of water and when the water boils see what the thermometer reads. If it reads 220 then adjust your recipes 8 degrees.