Welcome to the Do-It forums Darryl!
You'll want to add the uv blast to the raw plastic when you mix a batch. I haven't got a metric conversion handy to ounces but I use a small blob of the powder about the size of a regular paper-match head per 4 ounces. I add the uv enhancer to every batch of plastic I make regardless of the color.
I don't feel it does anything to better a fluorescent color. Both the fluorescent color and uv blast respond to the invisible uv portion of white light. Under a black light the uv enhanced, non-fluorescent, plastic will appear to glow about as bright as a fluorescent color without the enhancement. The difference is the fluorescent colors show their color while the uv blast is generally invisible to human sight but will show up as a blue-purple glow when exposed to black light.
You can tell when you've over-dosed the uv blast in plastic by the bluish tinge it will give the plastic. If you are seeing this cut back by about 1/2 of what you added that caused the plastic to "blue" and try again. Personally I think most people add too much. A little of this stuff goes a super long way.
Again.....welcome to the site. Enjoy!