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.