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Honestly, I'm not going to even consider Zanshin merits, because Store TP merits and Meditate recast should not be left under max. I don't care how good this has proven to be, you would be losing out in a big way should you let either of those fall. Store TP is not an option to not fully merit, you're holding yourself back from easily obtaining a 6-hit with mediocre gear by having that fully merited. I could see an argument for Meditate recast, but over a longer period of time, you'd probably lose out on a great deal of TP. Let's just compare unmerited to fully merited, over a 30 minute fight, shall we?

Over the time of 30 minutes, you would get 10 meditates in without any merits. Assuming you have Myochin Kabuto and Saotome Kote, you would range between 140-180 TP per meditate. I realize 180 has been only documented rarely, but I've seen it myself and can vouch for it's existence. It's most likely 140-160TP, which seem to be about 50/50 to be fair. Let's just round that off to 150 TP per meditate, so you'd obtain 1500 TP, or if you WS every time you get 100, 15 weapon skills.

Now with full recast merits, you would get 2 more meditates in, for a total of 12. 12x150=1800. That's 3 more weapon skills in the time of 30 minutes alone. Imagine that in the scale of a merit party, which can last multiple hours. I think you see what I'm getting at. Unless your accuracy is just plain terrible, I'm not seeing Zanshin merits being anything other than counterproductive.