Explosive arrow. Empower vs slower projectiles

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Matov wrote:

But he says that 20 quality 20 level gem increases damage for 29% while gem has base 29 % more damage mod.i mean 29% more damage + 10% increased Damage must make more than just 29% more damage,right?

An example with simplified numbers:
Gem gives 30% more damage + 10% increased damage from quality.
On tree and gear you already have 400% increased damage.
Your actual damage increase will be: (1 + 30%) * (1 + 400% + 10%) = 6.63 = (1 + 32.6%) * (1 + 400%)

So if you already have 400% increased damage, an additional 10% increased damage is only 2% more damage (and 2.6% when multiplied by 1.3).

thank you all for your help,i will pick slower projectiles over empower 3 lvl but will change to empower 4 if i ever get it.