Um - not sure about that - I use the onset of the sparking/lights out to mean that I'm below my safe depth. After I take damage, I go down to a deep depth - like 140m or so. If at any point i see the 'sparking' (and my CE says 'we are taking damage sir'), I note the depth and quickly blow ballast. From now on, I can't go that deep. If you are getting the 'sparking' effect, you are probably taking more damage, and hence reducing your safe depth. In short, I wouldn't advise hanging around to count sparking times because...they will get longer
