66" is my favorite length for D bows and I've built quite a few of them that length. 3" less that nock to not length is what I always use, and I seem to always come out just about right on for brace height and shootability. Finished a bow yesterday that was a bit different, but that measurement still worked. Bows is 67" and I made the string 64". However, it's a forward riser, which raises the brace height a theoretical 1" to 1 1/2". Brace height first time strung came out right at 7 1/4", twistable up or down without going too tight or too loose. Took it out to shoot and the first arrow out of the bow was a center kill zone shot at 17 yards. In years past I've had a number of Hill Cheetahs and again, 3" short has always worked.