Received Morse Code signal-to-noise ratio?
Personally, I'd use whatever filtering scheme the actual receiver uses. If the narrow-band CW filter is exactly this, or even close, I'd use that. Then try to automatically (or manually) determine the regions where the signal is present. Measure the average power in the "off" regions, and that will be N. Measure the average power in the "on" regions, and that will be S+N. S = S+N - N, and then take 10*log10( S / N ). This will give you S/N ratio for the bandwidth of the narrow-band filter.