I've heard (And personal experience backs this up) that the secondary proration factors in even if it's the first hit, so your first hit factors both numbers. For example, in all my experiences with 6D, even if I started the combo with it, the proration was SHIT TERRIBLE. I would rarely get more than 2k damage. Which, actually, makes a lot of sense if the secondary proration, 60, is applied to it. If only the initial, 100, was applied, I should be able to get some pretty unbelievable damage off a 6D starter... but I can't. Not only do I not break 3k, like I do on 3C, j4C, and 2B, I barely break 2k.