Calculate the SNR of a signal

Hi everyone, i've got some questions about calculating the SNR of a signal.

I want to evaluate an EOG signal. The problem is that I don't know the quantity of noise that there is on the signal and I need it to evaluate the efectiveness of some filters. Some people told me to do the FFT of a specific homogeneus noise zone with N-points of the nearest power of 2., secondly calculate the IFFT and then the SNR with the formula 20*log*RMS(OriginalSignal)/RMS(Noise).

Anyone knows if this is good? I made this but i'm not sure about this idea...

Thanks.