I do like the concept, but as you state it if a ENY 40 craft, shoots down an ENY 5 craft, The eny 40 craft gets the equivalent of 35 kills? Umm...no, I don't think so.
To modify this somewhat, how about this equation: your ENY / his ENY? It would make the same situation previously, work out that you got 8 kills if you were the ENY 40 craft shooting down the ENY 5 craft. If the situation was reversed and you shot down the ENY 40 craft from your ENY 5 craft it would score as 1/8th of a kill. I think that works out a little more balanced than your original suggestion.