If you want a dead nuts on reading the only way to get it is with a digital temperature meter and a thermocouple probe placed in the coolant as close to the thermostat as possible. I have seen new after market gauges etc be off by as much as 40+ degrees for a number of reasons and they normally go further out of calibration the warmer the coolant. infrared meters are only good for an indication unless they are calibrated to read the specific surface they are being pointed at.