I posted this in another thread. There are TWO different 300 caps. The damage increase cap and the damage modifier cap. I'm just going to paste this here as well as it is what OP is talking about...
For the
damage increase cap
If max Damage Increase for items = 100
If 120 Tactics then (120/1.6) + 6.25 = 81.25
If 120 Anatomy then (120/2) +5 = 65
If 150 Strength then (150*.3) +5 = 50
For a total of 296.25 which means its impossible to hit 300 here unless you have LJ.
20 LJ (Skill Level/5) *if below GM* will add 4.0, reaching the 300 cap.
@Finegold In your situation...
DI from items = 100
100 tactics = 68.75
0 anatomy = 0
150 strength (assuming) = 50
That means you're at 218.75 of 300 of the damage increase cap.
What King Greg posted for the damage modifier cap is right on