Relative yardsticks for Toughness and Armor

There is a lingering concern I have with running a supers game regarding damage output verses toughness and armor. At first glance you think a high parry hero can work. Then you realize that once an attack hits, that hero with a toughness of 5 is going to be pasted immediately. Even more so as that landed hit likely aced so the villain is going to get an extra D6 damage added to their regular 5D6 attack.

On the flip side, I can see some players having such a high Toughness/Armor that it'll take a tank round to even make them flinch. Again there it'll be this never ending arms race to overcome high defenses. With exploding dice things can get especially chaotic. So I am considering curbing the high toughness (and armor) some for a street hero game.

I'm considering having a general yardstick for my players to adhere to. If they are a hero that can be hurt by bullets, total Armor and Toughness should be between 8 and 10 points. If they are tougher and relatively bullet proof, their combined Armor/Toughness should be 10-16. Anything over that I'm going to push that they buy other powers like Resistance instead. It's not perfect but I feel with these guidelines I can throw villains at them that will be a challenge while not risking completely one-shotting them constantly.