Find Stopband Edge Frequency
In general there is no standard way to calculate the stop band edge, other than specifying a desired stop band attenuation and checking at which frequency this attenuation is first attained (for a low pass filter).

For filters with an equiripple behavior in the stop band (e.g. FIR filters designed by the Remez exchange algorithm, or Chebyshev II and elliptic IIR filters), there is a clear stop band edge, which is the lowest frequency (for low pass filters) at which the attenuation equals the minimum stop band attenuation (attained at the local maxima of the filter's magnitude response in the stop band).

However, your filter looks like it has a monotonic magnitude response, so the definition of the stop band edge is arbitrary.