[bojan]

For SDR reading of -30dB (noise floor is at ~-60dB), I had -61.3dBm CW at input (75ohms).. since I do not have SG that goes that high frequency-wise, I used tracking generator in zero span mode (then I can reach 1.6GHz), with external attenuator to supply the test signal level at SDR input.

On the screen plot it is visible how the recorder (SkyPipe, detector set to peak) has shown the drop in audio level when carrier drifted out of audio frequency range (gray area on spectrum plot), set by sample rate.
So, cal factor for my case is -31.3dB.

Which means the SDR level reading is in dBuW..

With additional 40dB in front od SDR (pre-amplifier) the noise floor will be at ~-130dBm (to be confirmed.. because there will be some noise from preamp as well).. which is pretty good receiver all in all..