Reduced Mana reserved - how does it work?

Question

How much mana will "Reduced Mana reserved" reduce from Spells that reserve Mana?

Here is what I mean: http://pathofexile.gamepedia.com/Reduced_Mana_Reservation An Example is Sovereignty: http://pathofexile.gamepedia.com/Sovereignty Putting points into it nets you 14% reduced mana reserved.

Let's say we have 10% reduced mana reserved

When you cast Purity of Lightning: http://pathofexile.gamepedia.com/Purity_of_Lightning Which reserves 40% mana.

By how much will it reduce the reservation?

• by -10% additive result: 30% mana reserved
• by -10% multiplicative result: 36% mana reserved

Incase it was additive: How does it work with a second aura? Let's say #2 is 20% mana reservation

• -10% of 40%: 30% mana, and 20% from the other, result: 50% mana reserved
• -10% of 40%: 30% mana, and -10% of 20%: 10% mana, result: 40% mana reserved

Answer 1

Reduced mana reserved can be tricky. You can try out this Reduced Mana Reserved Calculator to get a better understanding of how the math is applied.

In short, it takes the amount of mana set to be reserved (10% of your total) and chips away a subpercentage. But some things might not be as you would expect.

Here is a practical example:

I have 41% reduced mana reserved passives from the tree. I have a lvl 17 reduced mana gem, which gives me a 0.74 mana multiplier on the reservation. I want to know the exact calculation how much percentage a 60% and 40% is going to reserve.

So far i expected it to be: 60% * 0.59 * 0.74 = 26.196, which seems to be wrong.

I checked the source code of the mana reservation site linked before: http://fogz.mine.nu:8080/poe_reserved_calc/

There, you can see that they first multiply the aura reservation with the mana multiplier of the gem and then round down:

60%*0.74=44.4% -> 44% reserved 40%*0.74=29.6% -> 29% reserved

Then this reservation is multiplied with the passives and rounded up:

44%*0.59=25.96% -> 26% reserved 29%*0.59=17.11 -> 18% reserved

This seems to be the correct calculation.

Assuming this is correct, it is funny, because you actually need a level 20 reduced mana and at least 0.41% reduced mana reservation passives to rock four 60% auras on mana, which is exactly what I hope to achieve.