You can find out quickly on your calculator in 4 easy steps.

1. First find out enemy base defence for the depth you want to figure out your desired amount of bonus for. An easy way to do this is to compare neutral and weak damage with a high-damage weapon such as Dark Briar Barrage. You can use this chart. Base defence = weak - neutral. For depth 24, this is 65.
2. Remove the appropriate amount of defence from the weapon's damage. If you're looking at the weakness damage of a specialized weapon, this will just be adding the base defence to the damage. (For DBB, this is 283+65). For a pure normal weapon, add twice the base defence. (For Nitronome, this is 217+130.) The result won't be completely accurate, but it'll be pretty close. You now have the weapon's pure damage.
3. Add 4% for each level of damage bonus. If I want to check max damage for DBB, for instance, I'll do 348 * 124% = 432.
4. Subtract the defence again. 432 - 65 = 367 damage vs wolvers for DBB at max damage.

In formula form:
((2 x Weak - Neutral) x (100% + DamageBonus x 4%)) + (Neutral - Weak)

This falls a bit flat for weapons with low amounts of damage, though. Split-damage swords is also a can of bees for itself.

So it's basically only works for DBB, is what I'm saying.