I find if I shoot too close to the screens my (Pact)misses a shot now and then. 6-8" seems to be best but I'd imagine that would vary from one chrono to the next.
The easiest way for me to get perfect setup is to set my (100 yd zeroed) rifle up on the rest, aiming at a 100 yd target w/bolt removed (or just the upper if an AR). I then set up chronograph by sighting backwards down the bore of the rifle while centering the screens @ desired height.
Edit to add: I check velocities at the same time while shooting for groups.
Regards,
hm