The funny part about your question is, I don’t consider myself any type of gardening whiz whatsoever. I just know when I apply the castings, EVERYTHING grows.
I’ll tell you what I do, and you can try and figure out the math….
I don’t measure anything when using castings. I also don’t have a regular feeding schedule. Basically, I start my starts with a handful of castings. When they look like they can use a perk up, I give them some more castings.
When I’m not feeling lazy, I’ll brew a batch of Worm Tea…using my large rubbermaid wheelbarrow, A sump pump to oxygenate the liquid, Castings in a breathable bag, and dry molasses added to the water.
10 gallons of castings can feed quite a few plants. If you brew tea using the castings, they will go even farther. You may want to consider doing both and seeing how you feel about your results.