How does ruthless get calculated?

I'm sure this was answered somewhere but I tried a few searches and didn't really find what I was looking for.

I'm looking at upgrading an armor for my Assault and considering one that has gold ruthless (charge attacks deal +30% damage). How does that actually get calculated?

Say my assault typically does 4000 damage normally and 5000 for charge attacks. Does the ruthless bonus add 30% to the entire thing (so 5000 * 1.3 = 6500 damage) or is it just 30% on the extra part that comes with the charge attack (so 4000 + (1000 * 1.3) = 5300 damage)? I know there will always be some variation but those are decent rough numbers for what my character seems to do currently without that bonus.