I used the pest information from the Wiki's Campaign section as my base. For the average 5* stats, I went into each profile individually and plugged in all the numbers, one by one.
What tipped me off to this sudden drop off in Defense Value is when I tried to kill a fresh, 2* & 3* Flower Knight against 6-5's 1255-Atk pest, once with a Bracelet and once without one. I was kinda expecting more out of an additional +600 Defense Bracelet.
It certainly varies with the stage, but Dahlia's Attack Reduction stat is definitely saving our Flower Knights a lot more than a Defense Boost against the Pumpkin Pests' 8000 Attack..........I mean, we'd need 16,000 freak'n defense if we wanted to negate that! I can only muster up about +50% Defense boost at best + Equips, which comes out to 5000-Def at best.
If I'm not wrong about this, both Attack and Damage reduction abilities are a hundred times better than +Defense% abilities, in terms of HP saved.
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Yeah, atm, it looks like the best value of Defense comes from bolstering the damage from Counterattackers.