Probably the best way to "calibrate" your dash temperature gage is to note where the needle likes to stay once the engine is warm. If the T-stat is controlling and not pegged open (overheating), then the stamp on the T-stat (180 or 190 F) is likely where the needle is. That said, there is variation between T-stats and no T-stat controls an exact temperature. Under higher thermal loads, the T-stat must open more, and that only happens if the coolant temperature is higher. Thus, it will control a little hotter under load (racing up a mountain road on a hot summer day). If you have a T-stat out, test it in a pot of hot water. I got amazingly consistent readings shooting at in underwater w/ an IR gun against a thermometer in the water. Note that an IR gun views a larger area than just where the red laser dot hits, so get it close enough to fill the view circle with the hot surface. Harder to get consistent readings shooting at the engine's head or T-stat housing.
