Confusion in usrp_spectrum_sense.py

Hi all,
Could anyone explain the following statements to me ?Thank you !

1)log = gr.nlog10_ff(10, self.fft_size,
-20math.log10(self.fft_size)-10math.log10(power/self.fft_size))
What equation does this statement equal to and why ?

2)As indicated in the script, what should we do to deal with the
overlapped data because of 0.75? Our purpose is to obtain the correct
data for further signal processing .