Very Lights fizzle out, so conclude the Status Check Phase of each Attack Turn by checking each Very Light marker on the Turn Track. The reason those markers go on the Turn Track is to make it easy to keep track of how long they've been burning. The turns since the Very Light was fired is a modifier. If the flare burns out, remove the Very Light marker (and adjust the Activity Level at that dam).
So if I read this correctly, when the Very Light Burns out... you REDUCE the Activity Level by 1?