Self Sacrificing node not working properly in POE 2

I'm playing an oracle, the self sacrificing node improves reservation efficiency by 40% for minions.

Vaal Guard specters reserve 50 spirit base. After selecting the node they reserve 42. Shouldn't they reserve 40 after the 40% spirit reservation improvement?

...or is my math just not mathing?

Isn't it a 20% improvement?

The reservation efficiency math is a little weird, but I would think it should be at least that?

80% of 50 is 40 though. And my gem is showing a cost of 42.

Seems like I'm not the only one having this issue. And it doesn't seem to impact all the minion skills uniformly.

Efficiency is not a linear calculation. The more efficient you get, the less each point of efficiency will affect your costs (and as you go below 100%, the cost gets exponentially higher instead)

A skill with 100% increased efficiency has it's cost cut in half.
A skill with 200% increased efficiency has it's cost cut by 1/3.

A skill with 50% reduced efficiency has it's cost doubled.
A skill with 90% reduced efficiency has it's cost multiplied by 10.

To calculate with efficiency in mind, just divide your cost by your efficiency rating. (Note: You start at 100% efficiency, so a 20% bonus becomes 120% Efficiency)

With 20% increased efficiency and a cost of 50:
50 Spirit / 1.20 Efficiency = 41.666 or 42 Spirit rounded up.