Please, read carefully : In short, the problem is that none of those bonuses seams to work properly, or they work quite incoherently (imo) :
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Gem : Gain x% Focus on Damage Dealt (works incoherently)
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Gem / Cursed Bonus : Focus Gain increased by x% (doesn’t work)
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Gem : Gain x% Focus on Focus use (works incoherently)
And apparently, this specific bonus is the only one working for Focus Gain :
- Cursed Bonus on weapon : Gain x% Focus on Damage Dealt (will work even if it’s on your main weapon and you are using your secondary weapon)
I don’t know yet about Gain x% Focus on blocked attacks, or Gain x% Focus on “non cursed” enchanted items, there are probably others to test, but i was focusing on those that would help an archer build to be efficient with Focus imo.
Now, for more details, and after extensive tests, each one starting with the same amount of focus, and ending after shooting successfully 10 arrows on the same enemy, i will gain EXACTLY the same amount of focus if i have equiped :
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A main weapon with a cursed bonus “Gain 7% Focus on Damage Dealt”, and i’m shooting enemies with my secondary weapon, a bow with a gem “Gain 2% Focus on Damage Dealt”, a pant with a gem “Gain 4 Focus on Focus use” (which give me 15 Focus points, oddly), or …
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The same main weapon, bow, and pant, but adding a hat with a gem “Focus Gain increased by 12%”, and a cursed bonus “Focus Gain increased by 29%” (so total Focus Gain = 41%). Conclusion here, Focus Gain of 41% didn’t change anything to the equation.
And the crazy thing is that i have a bow with a gem “Gain 1% Focus on Damage Dealt”, which is less than my other bow (same one, a short bow) that gains 2%… the bow with 1% gains MORE focus than the bow with 2%. Let it sink for a second 
Here is a synthesis of all my tests, just note that for the last one, i had gain one level and used my points on Focus, so this last test is not really a reference, but it gives an idea.
I think it’s really important to fix this, as it can break any build that heavily depend on focus gain, and i hope this report will not get lost in the mass.
As always, many thanks to you all! You are really the best!
Edit : I did plurious consecutive edits, precising, correcting, adding details and the image showing the comparisons.