LimeSDR-USB Rx amplitude issue

I tried to use LimeSDR to reproduce the function of the simple spectrum analyzer and referred to the following flow diagram, but found that the signal amplitude displayed by “Frequency Sink” was different from that measured by the spectrum analyzer.

1.Transmit with a hand-held signal source at 900.5MHz@-40dBm;

2.In both “Frequency Sink” and “Vector Sink”, the Rssi is -68dB;

3.For the same signal, the spectrometer showed an Rssi of -39.9dBm;

4.What calculations do I need to get the correct Rx-amplitude through LimeSDR?

Or how to compute the power of a signal, in dBm?

With a sin wave you have

y=Vp* sin(2* pi* f* t)

Vrms= Vp/sqrt(2)

Pavg= Vrms* Vrms/R


R=50 for many antennas and coax cables