mana cost of skills

I'm a little confused about the mana cost of skills. as i just respecced and the result was not as i expected

my setup was:
incinerate l19 (8mana), faster prj(110%), faster casting(120%), fire pen(140%), spell echo(140%)
which comes down to 8*1.1*1.2*1.4*1.4=20.70 mana and ingame the skill costed me 20 mana to cast

now my incinerate hit lv 20 and now costs 9 mana, so i invested in 12% reduced mana cost of skills
so the cost of my skills should only be 88%

i calculated the cost of my lv20 incinerate with my new passives should be:
9*1.1*1.2*1.4*1.4*0.88=20.49 mana, however ingame it now costs 21 mana per cast even tough the cost is now less than before

could anyone help me understand this mechanic better? cause i have no idea how this works anymore

.

It's due to the way reductions like these are calculated. The game finds how much 12% of the original value is, rounds it down and substracts it, that effectively means rounding up.

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raics wrote:

It's due to the way reductions like these are calculated. The game finds how much 12% of the original value is, rounds it down and substracts it, that effectively means rounding up.

okay that makes sense, thanks