An offset (not slope) calibration is possible using the RC-5 software. With the device connected to a USB port (and the software permitted to "make changes to this computer" despite the "unknown publisher" or missing certificate): Click "Parameter Set" (in toolbar or Settings menu), then enter the required delta (negative to decrease the reading) as "TemperatureCalibration" near the bottom of the second column, then click button "Save Parameter", confirm and Exit. Note that although the Up/Down arrow buttons change the offset by a full degree, you can edit to the displayed resolution of 0.1°.