I believe BW recommends a brace ht between 8" to 9" (measured from the deepest part of the grip). I keep my MA braced at around 8 3/4", & every BW recurve I've owned has shot great w/a 9/16" nock height.

Your optimum set-up may vary from mine so don't be afraid to experiment to see what works best for you. IMO Black Widow bows are among the most consistent out there, but we're all a little different in the way we shoot em.