you cannot get IC with a combination of dex+reduction gears since they apply differently and dont stack. (etc, 135dex and berzebub=/=150dex. (but your cast time would still go down)
150dex=100% so...15dex=10%
last thing: if this is too confusing, there should be some DETAILED IC guide if you search it up telling you how reductions/dex affect each other