Cutoff frequency of lowpass filter
The correct way to actually find the cutoff frequency for a low pass filter would be to find the frequency at which the gain of the filter is 1/sqrt(2) times the gain at dc frequency. 

$$Gain (dB)=20log(\frac{A}{\sqrt2})$$
where A is the gain in V/V at dc frequency (not dB). Substituiting respective values from the graph you have, you should be able to get fc.