Question on units using USRP and

I’m a bit confused on what exactly is the input to the FFT when I am
usin I’m looking at a specific channel in the TV
band spectrum, and that signal is run through the ADC to be represented
by different values in voltage. We then these numbers run through a
stream_to_vector block before the FFT. So the input values to the FFT
block are then in terms of volts, correct?

I’m trying to convert the output of the FFT (after running through the
magnitude squared block) to dBm, but I’m still not sure what my units
are. Reading some of the other posts here lead me to believe that the
magnitude square is actually proportional to the power, so why does
taking the 10 * log(x) of all the values in the FFT bins seem to be
inaccurate compared to a hardware spectral analyzer?