The best estimate of the magnitude of an event that can be made with a SEP seismometer system is using the Surface wave magnitude formula (Ms).
Ms=log (Vmax / 2π) +1.66 logΔ + 3.3
Vmax is the peak velocity amplitude at period ~20 second (μm/sec). Vmax can be estimated by measuring the peak amplitude of the surface waves recorded by the SEP seismometer (with data filtered with a 20 second low-pass filter). You can determine the sensitivity of your seismometer empirically by monitoring many event magnitudes. For an amplifier set to x100 gain you should get a response factor of approx 50 counts/(μm/sec). Δ is the distance from the earthquake (degrees). This formula should work for events shallower than 80 km and in the distance range 2° ≤ Δ ≤ 160°.