Neither. Since the skill gets both phys & void tags any untyped melee damage (most weapon implicits) are split evenly between the tags. So if you had a weapon with +100 melee damage, that would give +50 phys & +50 void (plus the additional +8 for a total of +58 void).
Wait, that’s true if its Flat Melee. But it looks like he was asking about %Increased Void. So wouldn’t THAT just increase the Void flat damage already on the weapon?
One thing’s for sure. I did not know that adding the tag would split the generic melee damage or spell damge on weapons. It makes sense but I hadn’t realized it. That’s nice!