To determine the permanent offset for an individual device you need to compare it, using a 0.0 PPM offset, to a known stable frequency source. The ideal tool is a service monitor but for most monitoring purposes receiving a continuous transmitter over the air generally works well enough. NOAA weather radio or continuous duty trunking control channels are good candidates.

I haven't played with scope.py for a while, but if I remember right you can adjust the offset on the fly down in the lower left corner. Observe the signal of the transmitter in the spectrum display and adjust the offset until the frequency scale reads properly.

Brett