I was trying to achieve instant cast through VCT reduction gears/cards, not from stats. If you read my earlier post, my calculation shows I have stacked up 100% VCT reduction, but it seems that it doesnt. I wonder if my calculation is wrong, or at least 1 VCT reduction gear is bugged.
Because Seraphim Coronet is +4% and +6% cast time at 108 and 120 Int respectively according to the jRO description, which, in all likelihood, is more accurate than iRO's description.
Either way, there is no reason not to use +20 Int food where instant cast actually matters, and where it doesn't matter, just deal with it.