[renowner]

Honestly? Buckler Plus should just have its own deal, like...

"If you guard with 3M+, the next damage from Sword, Polearm and Axe weapons will be blocked, being reduced by 20%. This counts as a a 'parry', for skill activation purposes."

I like this idea significantly better. Grants you a unique parry skill that only activates if you have guard level 30 or higher. Chance is either flat, or maybe based on skill to reduce damage from certain weapon types by 30%.  Or maybe it should just give resistance?  
If you're already at guard 30, parry isn't going to be as effective at keeping you alive. But resistance could proc momentum destruction. . . Which I guess also sounds a bit dumb.

I talked myself out of it, the parry idea seems cool. 
Or. . .It just boosts parry rate by half of your guard level %?
So level 10 guard (1 momentum) is +5, guard 70 (7 momentum) is +35? That could also fit with the item's theme I think.