A plate thermometer (PT) that consists of a thin Inconel plate with the substrate of one layer of 6.4 mm thick and another layer of 12.7 mm thick ceramic fiber boards, was developed to calculate the incident radiant heat flux and equivalent total heat flux. However, by simply assuming 1-D heat transfer within PT, the equivalent total heat flux directly calculated using PT is lower than the reading of commercially available total heat flux gauge due to some uncertainties, such as emissivity, the error of temperature measurement, the lateral heat loss, the thermo-physical properties, and so on. In this study, with the calibration conducted under a cone calorimeter heater, two new methods of considering the uncertainties above were proposed, by introducing a new concept of ideal surface temperature of PT in an ideal 1-D heat transfer model or perfect insulation model, respectively. The calculated equivalent total heat flux using PT was compared to the reading of a commercially available Schmidt-Boelter type water-cooled total heat flux gauge for validation in a controlled thermal exposure scenario and a real fire scenario. With the method of employing the ideal surface temperature of PT in an ideal 1-D heat transfer model, the results showed very good agreement not only in the controlled thermal exposure from the cone calorimeter heater, but also in the real fire scenario. In spite of some discrepancy, due to its simplicity, directly employing the ideal surface temperature of PT in a perfect insulation model is still a useful method for fire engineers to characterize the changing trend and level of the equivalent total heat flux in the fire test field.