Afterglow Passive (Halp me plee)

[MeatSlinger3000]

So Afterglow passive reads: Increased spell damage; Doubled if you’ve used a skill that costs at least 40 mana recently

I was wondering how literally to interpret that last line of text. Can I spam, lets say, Lightning Blast enough times to spend 40 mana in order to proc Afterglow, or does this passive only proc if you use a skill that has a minimum mana cost of 40?

Also, adding proc/buff indicator icons in the game would be sick.

[Syncfx]

I read it as a single skill that costs 40 mana.

[ReimerhArts]

It is in a single use of a skill that costs at least 40 mana.