Shoot the arrow from 90 degrees a few times then move up on the string a little and shoot it again. Then go way high, like 3/4" above the 90 degree, just to see how the arrow behaves as you approach the extremes. Find the middle ground and mark the string. I use artificial sinew to make a knot where I want my arrow to go. You can use thread or whatever, or clamp on the metal type.