Re: How would I do this?
I can't speak for the OP, but Hugh's scenario doesn't seem so bad providing the campaign has firm guidelines, or even better yet, guidelines plus statted equipment/weapons. In the case of the latter, the DC 3 or 6 weapon may already be bought to zero Endurance or charges, leveling the advantages between both. I don't know. We'd need the OP to confirm that.
Doc, I don't think Hugh was saying "nay" to the idea, so much as pointing out the implications, which as he said, maybe the desired effect. Also, your example I think assumes +1 per DC of the attack. (Was that 4th edition?) Based on the OP, I was figuring very low DCs, so the plus one was to the final result -- not a multiplier.
Regardless of all that, a new player to the game has a vision of what he is looking HERO to do. There is a lot of know-how on these boards, but before we all pile on him saying it's, "It's not the HERO way," lets use that know-how to have the system flex a little and let it be the toolbox it is. Personally, I don't think what the OP is looking for is all that hard. It will require some tweaking, but as long as we're aware of the pitfalls and avoid them, it's possible.
If the OP is still around, we'd just need some more info. Right now my primary question is: how much control do the players have in designing powers? I'd also like to know what the campaign guidelines are.