Years ago someone told me you could grow in 100% worm castings and that the only reason why people don't that is because castings are so expensive. Sounded like BS to me, so I decided to test it out.
I got a dozen seeds, a bunch of plastic cups, and made batches of coir + worm castings with different ratios. I tried coir + perlite + 5% castings, then 10%, 15%, etc. I tried it again with peat moss, perlite, and castings to make sure it wasn't a fluke. The results were that 5%-10% showed improved growth, but adding more seemed to stunt growth. 15% was either no improvement or stunted depending on strain. After 25% none of the seeds would sprout.
I now keep castings at between 5% and 10%. Ten seems to be the magic number. It's safe for seeds, clones, and just about any plant I've toss it at. The NPK of the castings I used was 1-1-1.