In the EC client, you can look at your character's Attack tab and get the current damage range. Put this onto a hot bar for easy viewing.
Your maximum damage, regardless of anything, is 3x that maximum damage. You can get there using slayers, EoO, Honor, or whatever, but you will never exceed 3x that maximum.
For example: damage = 1-6 means max is 18
damage = 54-72 means max is 216
If you Lightning strike, then every so often you will get a critical hit which should be at this maximum. If not, then you're missing something.
To me, this is the easiest rule to keep in mind.
This seems to be most accurate by my testing, I hit the 3x max when I AI, which should in theory be the most damage possible (assuming a good RNG on the damage range). The neat thing about doing it this way is you can see the effects of potions and equipment directly in your status bar.
It also seems to support what Stupid Miner says about the 300% cap coming into play AFTER skills and items are figured in, leading me to believe that the 300% only applies to Arcane effects (EoO, Honor, slayer, etc.)
However, I disagree with Miners suggestion that EoO gives 50% damage increase which when figured with the 200% = an additional 100%. It seems to me that if you already have 200% then EoO is only going to add 50% to the total increase, ie 250%. That would explain why you are seeing more damage when you honor, Delbrie, you are actaully hitting the 300% at that point.
I could of course be wrong, but so far, every test I have done supports this. I too see more damage when I have EoO, slayer, and Honor active then when I just have EoO and slayer active as Miner suggested.
So, my belief = I have a base weapon damage of 18-22 on my soul glaive.
Then I have 100% increase from my equipment, plus strength bonus, plus anatomy bonus, plus tactics bonus. My displayed damage is 66-81.
So, with EoO, honoring, and slayer, AND when using AI, I should hit for between 198 and 243. When testing, all of my hits were in this range.
When I had only EoO and slayer, AND when using AI, I should hit for between 165 and 203. This was also true in testing.
When I used slayer only and with AI, I should hit between 132 and 162. Again, this was true in my testing.
I'm probably wrong somewhere, but my tests seem to support my theory.