Use this link to determine how much dex is optimal for your character.
https://docs.google.com/spreadsheets/d/1sPjFvpR40HMS8_1F7leojuz0hh1qCYVaRe_pkWjOyqw/edit?usp=sharing
The way you use this calculator is you find the casting speed of your spell and then look at the 3 dex amounts to the right of it. Each of the amounts is a breakpoint for you to hit. If you can't hit the breakpoint, then any of the points used are wasted, and you will not gain any advantage for using the extra dex and should use the points elsewhere.