In an argument with a guildmate about this.
His claim is that focii decay based upon the level YOU are.
My claim is that the focii is based on the SPELL LEVEL you cast.
Like if you have a level 80 focus of 50% and a lvl 85 focus of 45%, lvl 85 spells will trigger the 45% lvl 85 focus whereas anything under 80 (and possibly 81 or 82) level spells will use the 50% lvl 80 focus.
Would be nice to know how it works. I think I'm right as it explains why different focii trigger for different spells (or at least it seems that way)