If all else fails, use the "Auto" function in the Junction menu. That will junction the best magic to their best corresponding stats for optimal results. I usually use the Auto > Def setup as the basis for my junctioning.

It'll take a while to get used to what magic should be used for what. So again, go into the Junction menu, and select Auto, then pick either Attack, Magic, or Defense (whichever one you want to focus on). :D