The best way to calibrate is to get one of these.
Your thermometer may be dead on at boiling and freezing, unfortunatly, you aren't mashing at boiling or freezing. You need to make sure it reads right at 150-160.
Heat some water to 150, put both thermometers in the water, and trust the lab thermometer.
It's the best method I found so far. (It's also the method used by QA at a sausage plant I used to work at, for whaterver thats worth)