Relationship between FFT in dB and spectrum analyser output in dBm
The reference would be relative to the top of the dynamic range of the FFT block input, plus maybe a fixed offset.

If you want absolute power, you need to know the gain for all the components between your RF input and the FFT block, which is something you don't have readily available most of the time.

You can calculate the gain of the digital blocks, obviously, but for the analog components you need a calibration against a reference (which has conveniently been done already for the spectrum analyzer).

You have to take into account that the frequency response is flat for neither the RF parts (so you have to measure at different frequencies) nor for the IF parts (so you have to measure at different offsets from the center frequency.