Why TBN is a powerful but poorly designed ability.

[Starkbeaumont]

Lets say you have a TB that does 54000 damage in one hit. Lets say you have 45K HP. Mix of fending and slaying, some HQ food, whatever.
DADM = 37800
TBNDM = 36900, plus blood gauge.

If it does 54000 spread out over 6 hits of 9K...
DADM = 37800
TBNDM = 38250, plus blood gauge.

I don't understand your math here.
The DM mitigated attack hits for 45.9k. You have 45k+9k (TBN). so you survive with 8.1k
DADM mitigated hits for 37.8k. You'll survive with 7.2k

That is also true if you are looking at a multihit. TBN is just an extension of your HP

DM+TBN wins.

Now let's say the tank buster's base is 60k.
DM+TBN: 45k + 9k - (60k*0.85) = 3k HP left
DADM: 45k - (60k*0.7) = 3k HP left

Both are equal for mitigation. DM+TBN still rewards Blood

Tank buster base at 64k.
DM+TBN: 45k + 9k - (64k*0.85) = -400 HP. dead DRK
DADM: 45k - (64k*0.7) = 200 HP left. not so dead DRK

DADM is the winner.

So DM+TBN is not always better.