Spirit reservation efficiency for minion skills seems to work incorrectly

The reduction percentages granted by the reservation efficiency modifier seems to be off:
Most vivid example:
With a minion requiring 50 spirit for reservation (base):
- when reserved without any modificators, it uses 50 spirit (as it should).
- when used with the Lord of Horrors talent (that grants 12% increased reservation efficiency) it is 45 spirit, while it should clearly be 44 (50 * 0.88 = 44.00). Initially I thought that the bonus figure might be wrong it it actually grants 10% instead of 12%, but then i checked other options as described below.
- when used with further improved spirit reservation efficiency from Bones of Ullr (with 25% increased reservation efficiency), it becomes 36, while it should be 32 - if the bonus is additive (50 * 0.67 = 31.5); 33 - if it's multiplicative (44 * 0.75 = 33); or at least 34 - if we take the 45 of the above reduction (45 * 0.75 = 33.75).

It is true for other minions (with different spirit requirements) but is more pronounced for such that require a lot of spirit. You keep loosing 1 to 4 or even up to 9 spirit (in case of those that require 100 spirit) and it all adds up :(

Perhaps I am wrong and missing something?

Just in case smb else is wondering, I found a reply in another source that seems to explain the discrepancy.

Reservation is calculated as
Base cost/(100% + efficiency%), or
Base cost/(1.0 + efficiency decimal).

Therefore, in my above example, it seems to be calculated as:
for +12% improved reservation: 50 / 1.12 = 44.64 - this explains 45 I see
for added boots with +25% reservation: 50 / 1.37 = 36.496 - this explains the 36 figure.

In terms of an average user (like myself) it is so obscure :(
I'd say most players assume simple percentage reduction based on the description.

FYI - this is because if they did it as a simple % reduction it would be way, way, way too strong - i.e. 100% reservation efficiency = 0 cost spirit = infinite minions (lol) = game broken.

100% reservation efficiency = 1/2 spirit cost makes sense IMO, if you double your efficiency (+100% is doubling efficiency) spirit cost is 1/2 as much.