Applying ferocious strike from the Ferocity domain

[HangarFlying]

Ferocious Strike (Su) wrote:

Whenever you make a melee attack, you can designate that attack as a ferocious strike. If the attack hits, it deals additional damage equal to 1/2 your cleric level (minimum +1). You can use this ability a number of times per day equal to 3 + your Wisdom modifier.

Can I wait to declare my melee attack as a ferocious strike once I know I hit the target, or do I have to declare the attack as a ferocious strike (thus, expending one of my available uses regardless of outcome) prior to making my attack roll?

I am asking because an adventure I will be GMing tomorrow indicates that I can declare a ferocious strike on a successful hit, though after reading the actual description, I don't know if this is true.

Thanks.

[Elamdri]

You must declare the ferocious strike BEFORE you make the attack roll, hence the text "If the attack hits"

The rule of thumb in Pathfinder is that when you modify an attack or use a power that affects an attack, you make the decision before you make the attack roll. There are some things that you can chose to use after making the attack, but the text will explicitly tell you that if it's the case.

[HangarFlying]

Yeah, that's kind of the way I was leaning too, I just wanted to get a double-check.

Thanks.