In lieu of not being presented with a better method outside of the abstarct of Step Values (again these are not NOW applied when hitting multiple targets on purpose)... so I stand by Step Values still, sorry
And as per Star Fire - I like my rules - the CORE presents weak rules / so do as you wish herein
I don't mean to come off as too critical; I get that you've put thought into this. It's just that I can't help thinking that this whole step damage thing, whilst a good initial idea, is one of those ones that the more you think about it in relation to the rules as they stand, the more complex the interaction with those other rules is going to be if you're to preserve the nature of those other rules.