I'm sure I will mess this up, but I will try and explain how this works. Don't take this as being 100% factual, but the concept is correct. Heat stabilizer is typically a metal of some type that has been dissolved into a liquid and added to the plastic compound.
When the PVC melts it splits and produces a chloride that will attack the other parts of the compound. That's where the heat stabilizer comes in. The chloride prefers the metal and attaches itself to it vs. the other parts of the compound. Once a molecule has been attached to, it can not be used again. This is why additional heat stabilizer is needed, as you continue to heat your plastic, additional chlorides are produced.
The production of the chloride happens when the compound is heated. How much it produces is based on how hot it gets. Because of this, there is no "formula" for "x" amount of plastic. It totally depends on the original plastic formula (quality of ingredients) and what you've done to it.
Over time, you will get use to the plastic you are using, refine your heating techniques, and get a feel for when and how much stabilizer you need to add.
With that said, try adding 5-10 drops and see how that works for you.
I'm not a chemist by any stretch of the imagination. However, I have learned a ton conceptually about the different ingredients and how they interact as we've worked on our formula.
Hopefully this helps a little.
Jason