First you balance the backplate alone. It should require very little if any balancing, especially on alu spinners with machined backplates. If you have to drill a "series" of holes something is really out of whack. Check if the shaft hole is properly centered. I wouldn't use a spinner that has visual runout.
After the backplate is balanced properly, put on the cone, and rebalance the assembly. You can do this by removing material, or by adding material, or both. Adding material can be as little as a couple of drops of CA inside the cone, one the light side.