The problem about setting a dk "target" is the fact that the sitter's dk can range also.
For example: Louie. Louie, aside from Yuki (now gone) is the strongest sitter. He's capable of killing someone with, easily, 600 DK+.
Should a regular sannin come in in attempts to kill a 40 dk account, they would need, roughly:
30 Atk/Def (Base) + 25 = 55 Atk/Def Vs. 40 DK + 30 Atk/Def (Base) = 70 Atk Def.
The above is disregarding any values of attack defense, etc. that the opponent has gotten, or the sitter, due to oro kill points, etc.
So, the difference between 70 and 55 = 15. Meaning the sitter, the 25 dk sannin, would need only 45 Atk/Def to potentially kill a 40 dk account.
A genin, such as Jinzo (Osho-Kiba) would need:
2 + 30 Atk/Def = 32 + 38 (difference of 70-32). He'll need, roughly, 68 Atk/Def.
As it is, Jinzo has 213 Atk and 209 Def. Plus the 2, he's at 215/211 respectively. Assuming the target had the base of 30/30, Jinzo is on par with someone with 181 Dks (lowest stat of the two is acknowledged for margin of error).
We're looking at a rough formula of:
1) ADK (A's Dragon Kills) + 30 (Base Atk/Def) - BDK (B's Dragon Kills) + 30 (Base Atk/Def).
2) Add difference to both sides.
= Difference equals remaining Atk/Def needed to engage.
In substituting for my case vs. Rare:
(8 + 30) - (14 + 30) = ?
38 - 44 = -6.
+6 +6
44-44 = 0.
Meaning I'd need to have 44 Atk/Def to kill Rare.
Obviously there's lots of things missing from the formula such as a definitive value of that specified person's stats. I.E. Jinzo having 200+/200+ rather than 30/30.
The above formula is just to show Rare how sitters need to think when they fight someone.
Edit: Added Jinzo as example also.
Edit: Added "formula".