Everything I have seen usually shows the ratings on castings below 1 on N-P-K. I have heard it is more of a growth hormone type of thing than a fertilizer.

NPK values are NOT dependent on what you feed the worms and you won't be able to manipulate the end result, which is closer to humus than anything, rife with bacteria, which is what makes for a healthy soil - any type of compost can be high in various trace elements depending on the feedstock [some foods are high in boron for example, others high in sodium], but the NPK will always be pretty balanced in the range of 1-1-1 to 3-3-3, a soil amendment, not a fertilizer

bacteria work on the nutrients of mixed organics - N for them is like protein for us, Carbon for them is like carbohydrates for us - enough of each and they breed and multiply - the worms 'slurp' the microbially rich decomposing surface layer of the material. and do a poor job of digesting the bacteria, leaving castings that contain much of what went in, including live bacteria, which continue the process .... eventually the worms and bacteria together break down all the material [continually re-ingesting the castings] to a very stable humus

Bill