jonesy,
My Black Widow ended up being 5/8 above center and that did it. Took me a while to go above 1/2", as I kept insisting that 5/8 was too high, but when I finaly did, it turned out perfect. Funny, if I went an additional 1/8" above the 5/8" I was WAY Nock High. Go as high as almost 3/4 if you need to. If that does not do it, start over at 3/8. You may be too high and your arrow is bouncing off the shelf (giving you a false "nock high"). Rest easy tonight and try again tomorrow. You'll find the SWEET SPOT!