So dug a little further on this project. I decided to weigh some rotors I had laying around to get a an idea of how heavy a weight I wanted.
The HPI mini rotor weighs 335g
treats cdi za rotor 904g
stock e50 rotor 875g
I'm thinking 350-450g would be a good place to start.
Yikes... the first machine shop wanted 250.00. I'm def going to shop around.