Calibration of USRP wbx board

I want to calibrate USRP2-wbx device.In grc i just connected UHD:USRP
source block to wxGUI FFT sink.I connected signal generator output to
USRP and got the peak at particular frequency in GRC.
I gave -30dbm in signal generator,and i got -51 in GRC.
What is the unit of value(amplitude 51 that i got) that i see in grc ?

Please tell me any other way to calibrate the device.