How does "increased reservation efficiency" work?

Hi there, this might be a stupid question, but how does increased reservation efficiency actually work? The behaviour I was expecting is, that 10% increased reservation efficiency would reduce base reservation cost by 10%. So a max quality CoC would go from 100 spirit to 90 spirit. Together with "Efficient Inscription", which gives another 20%, I expected the spirit cost to go from 100 to 70, allowing me to activate a secondary spirit gem with a cost of 30 spirit. What I get in the game, is a max quality CoC going from 100 to 91 spirit reservation. Coupled with "Efficient Inscription" it goes down to 77, leaving me with a much less useful 23 spirit. Can anyone enlighten me how those numbers are calculated?

I see, thanks for the explanation. Was that a nerf compared to last season? I feel like before the switch from reduced spirit cost to increased reservation efficiency a full quality CoC was 90 spirit. But I might be misremembering.

it was this way since start. Pretty basic formula for cdr/reservations used in most of games.

Thank you so much, I'm terrible at math but you're extremely simple explanation helped me grasp the res efficiency math. Thank you again Mr. Seaman#4993 I was going through such a headache