Companion reserve..

So i have 100 total spirit, am using 2 buffs consuming 30 each and have 40 remaining. I have tamed a beast which reserves 47.4%, however i have 20% quality the gem and taken an 8% companion reserve efficiency node on my passive tree. 18% of 47.4 = 8.5.. but somehow i am 1 spirit short from being able to activate all 3, is this an error in calculation or am i missing something? even if you round the minion up to using 48%, 18% would still bring it to 40 spirit, i'm confused.