If the base endurance of every power has been reduced by 13.33%, doesn't this mean that any endurance reduction enhancement is also reduced in effectiveness by 13.33% since they modify the base value?
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No. The Enh go by percentage, so the effectiveness stays the same.
For example:
Power A costs 13 End.
The Enh reduces this by 20% so now the cost is 10.4
Now with the discount:
Power A costs only 10 End.
The Enh still reduces by 20% so now the cost is 8
If you look at the absolute numbers, yes the reduction dropped from 2.6 to 2 points but if the enh would still reduce the full 2.6 points then this specific enh would actually suddenly be worth more (26% instead of 20%) while others stayed the same (like dmg or acc)
The numbers I used don't reflect actual values in the game. Just for demonstration purposes