Fin you are 69 feet above IV's elevation. I've asked in this thread, what is the maximum effective damage of the upward blast in AH? I don't think anyone has stated on that. Pervert was at ~140 ft, double your altitude. I see it more likely to happen at 69ft than 140ft, however I'd imagine it should be more like 30 or 40 feet, just a guess there. I say max effective, in that AH only counts strikes when you fire gun/cannon when you actually hit, so max effective range in this case should be a blast that would cause some damage everytime.
We know this doesn't happen at 500ft so it's obviously coded into the game.