Mana Efficiency formula for idols is broken

[CaptainBreakfast]

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Mana Efficiency with Syncronised Strikes from the very long and inconvenient rogue idols does not match the advertised percentages.
I have an Idol with 15% mana efficiency with sync and my sync strike cost is 62. Logically, using that humongous monument should get the cost down to 52.7 mana cost. Instead, it somehow rounds up to 56.
The entire idol inventory can fit 2 of those inconvenient things. My second idol gives 10% efficiency. Adding them both (25% efficiency in total) should lower the cost from 62 to 46.2. Instead, it rounds up to 52. I understand that bonuses might not be additive, but how do you explain when we only use just idol?
Plus, considering we can fit only 2 of those in the entire inventory, just to improve QOL for 1 single skill, it feels very unfair and aggravating.

You can find a couple of photos in the links below
https://imgur.com/a/2kGW964
https://imgur.com/a/fj9uQoZ

[rinleezwins]

That’s not how mana efficiency works. You’re confusing it with reduced mana cost.

Here’s a reddit comment from an EHG employee from years ago:

"|So, Mana Efficiency basically makes your Mana count as more.

With 100% efficiency, if you cast a 90 Mana skill, your Mana bar would lose 45 Mana. The value of each point of Mana was increased by 100%."

Formula(not sure if still the same):

" mana cost = raw mana cost / (1 + mana efficiency) where, for example, 80% mana efficiency is 0.8 in this equation"

Source: Reddit - Dive into anything