Maybe of interest to ScottG and Fred Bramley. Both brilliant engineers!
You can not use a regular radio or an SDR unless they have been calibrated.
To do this, you need to generate a known RF voltage on the frequency you want to measure. ( Lets say it's 500 mV). You inject this into the radio and read the voltage. You then adjust the gain on the radio, on the same frequency, to read the same voltage. You now know that the radio is calibrated.
I'm not sure if there are many radios capable of this apart from those intended for this purpose.
All readings from non calibrated radios are for indication only and are not accurate.