I imagined it as dividing the damage before subtracting the armor roll, but either way, it really reduces the chance of doing significant damage. It's a fumble after all. Still, this is something that I'll keep in mind for future revisions.
I've done some testing my own, but no, I haven't actually unleashed this on any unfortunate players yet. I honestly made this just for the sake of making it, not actually expecting it to be as well received as it has been. So I'll likely keep tweaking it based on feedback.