As to the question on how much microbballoons to add, you would have to look at the specific weight:volume of both the resin and the microballoons you are using. To give you an example, I source my materials locally, for what I use a 15% mix by volume makes the product neutral (without hardware and finish), I have found that about 40% for smaller cranks and a little less for larger lures works best. The more you add the higher the viscosity the resin and the more fragile the bait.
For a couple of lures I have to inject the resin mix with a 25CC syringe. I opened up the tip to 1/4" and the mold has lines that inject from the bottom up to release the air. Inject very slowly and you get a perfect product with very little flashing and gate to cut off.