Grace Reservation Efficiency

Hello.
I just took Survivalist with the Mastery Point: Grace has 25% increased Mana Reservation Efficiency.

If I look at the Skill Gem it went from previously 50% down to 40%. Is it a bug? Do I have my math wrong (25% of 50 = 12.5 = 37.5%) - Rounding error?

I put the gem in a separate slot to check nothing increased the reservation, but that didn't help, still at 40%.

Its not a straight 25% reduction.

https://www.poewiki.net/wiki/Reservation

That has the formula used which gives you the 40%.

Thank you! Interesting, I have never noticed that before...

In simple terms, 25% increased efficiency means you can "reserve 25% more." So 100 mana can reserve 125 worth (or put another way, 125 only costs you 100 mana). 125 to 100 is a 20% drop, hence your 50 went to 40.