So in my server, level 150 is fairly easy to reach and I wish that I could make it so it starts getting some notable damage bonus at level 150 and reach the max at level 1000.

@zvn570 You can adjust the divisor in the config, but, technically, Enigma Resonator will never cap out. You will reach a point of diminishing returns, though

What's the offset should be if I wanted Enigma Resonator III to deal 89% damage at level 1000?

@zvn570 I mean, you're more than welcome to calculate it since the damage is directly correlated to the amount of experience you have.
Formula: max((log_e(numSouls * level + 20) / offset) - 1, 0)
numSouls = the number of XP the user has
Offset Note: higher offset = less damage
Default: 3.25f
It is worth noting that log_e is the Natural Log of e, which is about 2.71828

So I did 21.6775 as a offset but it ended up not giving any damage boost at all even when at level 1000.