WS modsFollow

Hey everyone, appologies if this appears a noobish question but i scanned the forums and couldnt really find anything to enlighten me on the ws mods.

So heres my question Regarding corsairs Slugg shot, when i click the link for this weoponskill it says the modifiers are 30% Agility, so does that mean it adds 30% damage for a certain amount of agility i add to it?

Its makin my head hurt (never was good at maths)lol

Exaple

Say i use Slugg shot without any AGI equiped and it does 900 (for arguemnets sake)
Then i add a certain amount of agility will it add 30% Damage (so 300) to the standard WS damage ? Making it 1200 ? Roughly~
Or how much AGI will i need to use to get to a 30% modification?

Any help is appreciated and i appologise in advance if the answers is simple but i just misunderstood it :)
Thanks ^^