OP calculation are already correct on 2nd setup.
with zen having 2 seconds long FCT, combination of temp dex boots plus +10 spirit king circlet should give faster cast than first setup.
alternative way on how to test the accuracy of 70% fct cast given by the circlet is by comparing any mid / top fenrir carded headgears with temp boots setup on zen skill.
if fenrir and temp dex proved to be superior, then current +10 effect granted by the circlet are incorrectly provide fct reduction bonus.
another way to test it, apparently level 5 sacrament and temp dex boots still gain slightly upper hands, by giving zen 1,25 fct seconds cut (remaining fct 0.75 second)
if sacrament + temp dex boots are about the same/ slightly better than setup 1, then you can safely conclude circlet +10 fct reductions are bugged.
another sample of skills to test are mage class and its subtree energy coat, which requires 5 seconds fixed cast time.
In case OP want to notice more apparent differences between the 2 setup.
OP can make 3 setups scenario, instead of 2 setup where the setup are as follows :
Setup 1 (Magic Essence 7 + Temp Dex)
Setup 2 (+10 circlet and temp dex)
Setup 3 (sacrament + temp dex)
Expected Results (Hypothesis)
Setup 2 (1.35 seconds remainder fct) should by far beat setup 1 (3.8 seconds remainder fct) noticeably
Setup 2 (1.35 seconds remainder fct) is slightly superior than setup 3 (2.25 seconds fct remainder)
Edited by SeiginoYuusha, 14 December 2018 - 03:12 AM.