I've never seen a negative Damage boost increase damage, which item did you see that with? Or was it something of your own creation?
Here is the explanation from one of Larian's Docs:
DamageBoost
This is a positive or negative number in percentages that adds that amount to the damage after it's been calculated.
E.g. if you fill in -50, it will first calculate the damage using Damage and Damage Range, and then it will cut both numbers in half.
If you fill in 200, it will double both numbers.
Damage, Damage Range
Usually, the inherited value should be okay. However, for special enemies, it is good to know how weapon damage is calculated.
DA weapon does between x and y damage. How code calculates these numbers is as follows:
base damage x = Damage level mapped, i.e. in Damage, fill in a number between 0 and 10
base damage y = Damage level mapped + Damage Range level mapped, i.e. in Damage Range, fill in a number between 0 and 10
Example:
Damage 2 and Damage Range 6 = weapon does between 2 and 8 damage (level mapped)
Damage 5 and Damage Range 3 = weapon does between 5 and 8 damage (level mapped)
This means that the maximum damage of both weapons is the same, but the second weapon will be more interesting because its minimum damage is higher. I.e. its average damage output will be larger as well.
