so you're almost always going to get 65% normal damage and often 130%.

Where do you get those numbers?

The damage of the cluster missiles is split into 50/50 for each volley.

683 dmg for a base missile + 30% dmg = 888 dmg -> split up in two volleys = 444 per volley.

Thats how the damage of Double Volley is calculated.

Edit:

Okay, now i got it. You meant 65% of a base cluster missile. *doh*