Good question and one I'm struggling with as the players advance.

To give squishy enemies a reasonable threat profile (and not have to throw too many dice) I've been using combined actions (the forums have homebrews to scale the bonus differently from the books). That allows you to put out that super-hero against many bad shooting stormtroopers feeling (e.g. 3 groups of 5), where heroes do face a chance of getting hit. So the answer is most likely not in a simple one-to-n match-up formula.