Hi, I'm trying to use Proteus to simulate a basic low-pass RC filter. The filter is built and behaving as it should, and I have a frequency response plot of dB gain against frequency. The next thing I need to do is be able to calculate the phase shift of the filtered (output) sine wave. I've plotted a second frequency response graph which shows the phase, but I can't seem to use it to get specific values. I can get rough values by just looking at the graph, but I need more specific answers.

Can anyone show me a way to actually calculate the phase shift of the output?