Is this controller capable of being two-point calibrated? A one-point calibration is certainly better than nothing and does ensure that the controller is fairly accurate near the single calibration point but it does nothing to ensure accuracy further from the calibration point. You're setting the intercept but not the slope. A second calibration point lets you set both.
If you can only do a one point calibration, I would recommend the boiling point of DI water (corrected for elevation or better current atmospheric pressure) instead since it is much closer to mash temperatures than is the freezing point of water. Another option is to do a two point calibration and develop and calibration curve even if the controller itself isn't able to be two point calibrated. Just note the temperature the controller measures when placed in a DI ice slurry and a DI boiling water bath. Plot the controller reading on the y-axis and the true reading on the x-axis and connect the two points with a line. Then any reading taken off the controller can be easily corrected. Find the temperature the controller reads on the y-axis and move sideways until you reach the calibration line, then travel straight downward until you reach the x-axis. That is the corrected temperature.
The same thing can be done with multiple boiling points such as pure grain alcohol (actually only 95% ethanol and balance water) which boils at 172.6°F; a very useful third calibration point for homebrewers.