It's really going to be matter of personal preference, depending on the outcome you want. The more mica, the more the light will be obstructed. I would suggest starting with a little and adding as needed. When I pour blanks I want there to be enough mica to hide the brass tube when the resin is turned thin. I check this by holding a stirring stick (chop stick) against the inside of the cup wall after the mica is mixed in the resin. I want the stick to disappear in the mica as I move it away from the cup. That might give you a base line as to how much you have mixed. I'm guessing the lights will shine through the resin at that point as long as it's not too deep in the channel. If you have extra lights you might want to make a test pour. Sounds like a cool idea. Good luck!