Three drops a liter should suffice.

The earthworm castings are a very good choice. I think it has a little NPK value and somewhere in the neighborhood of ten trace elements, maybe a few more I'm not sure. So yes, you can use the castings for both vegging and flowering as they won't interfere much, NPK wise.

That site you listed has some good stuff. That Rootgrow Fertiliser has beneficial fungi that'd be good for both vegging and flowering. The J Arthur Bowers Sulphate of Potash and Superphosphate are good, too. Those two together would make up excellent phosphorous and potassium values for flowering. That Maxicrop Organic Plant Growth Stimulant also looks like it'd make a good additive. And the Maxicrop Organic Seaweed Meal wouldn't hurt, either.

The Epsom salts are important because they contain one of the top five most needed nutrients for cannabis, and that's magnesium. It's probably one of the most common deficiencies, so it is important to at least have something containing magnesium in the event the plant(s) aren't getting enough.