User blog comment:Shinhwalee/Guide to Power Leveling Heavy Cruiser (CA) in World 3-2A/@comment-212.183.51.128-20150321122443/@comment-3502824-20150325193639

Here is a question:

Is your Furutaka's Firepower maxed?

Because if not then you should not be using Double Line.

In fact using Double Line decreases MVP gain somewhat due to damage decrease.