[Shujin]

5% for a 1M guard is enough. I rather have it be 5% less damage for everyone though rather than a damage increase. More shield protectors and all that.

Maybe 5%*M invested to a max to SR (so 3=15%)

I be generally pretty on the fence on everything thats % damage increase, considering the current state of the game. no matter how small it seems. Even more so on defensive options that are also just something small as a talent.

Edit:
Also not stackable...As much as a roman shield formation sounds fun on Paper, having a full party basically stand there for 100% reduction guarding sounds stupid as fuck.