Expression And Time ? Pixel Anylyzer

Hey Guys

This is what I am doing.

I have a fight scene which is taking place during a thunder storm.

My task is to comp further rain (particle or live action plate, does not matter) in to each shot to bring a level of consitance to the shots.

Now as I said above it is taking place during a thunder storm,, So I have used the "pixel anylyzer" to mapp the luminance values for when the lighting strikes,, I then apply this information via an expression (nothing special) to the "brightness" node.

one last note "the lighting flashs for only one frame for about 90% of flashes".

My question is, What sort of expression would I need to use if I wanted to add 1, 2, or even 3 frames of ease in and ease out either side of the lighting flash using the data from the "pixel anylyzer"

Regards
sydway