I'm not sure I follow this bit, or you may be making assumptions about how damage works that aren't true.
That was the case...
Currently foes have attack and defense power, just like heroes. So I don't set a damage range per foe, the calculations just work out based on relative power, plus some RNG. So while I guess I could split the attack powers between physical and elemental, I don't know that the numbers I'd have to work with would be very intuitive. (In other words, halving the power doesn't necessarily halve the damage.)
...and now I understand how the table is set
I'd be far more likely to just add a percentage for how much is elemental, but then I've gotta adjust for resistance, damage reduction, and all kinds of stuff, all in fractional amounts.
I suppose an important bit is what the player sees. So, for example, if a player sees a creature doing 50
damage, he expects to see the elemental damage be removed, if he has 100% protection. The worst case would be having the damage as a single value, with an obscure percentage into play, resulting in the player not being able to 'feel' the elemental resistances.