Set it up 10' or so in front of the muzzle.
Have it setup in the same plane as the bullet (level if the target and rifle are at the same height).
The closer the bullet flies to the sensor, the more accurate/consistent the numbers will be (too close and you will be in the market for a new chrony ;0 )
The absolute number you get is less important for load development than the relative change in velocity you get for a change in your load. For example, did seating the bullet .001" closer to the lands increase or decrease the velocity spread I get for this load? Or, why didn't I get any velocity increase for an increase in powder? (powder too slow for barrel length).
The actual/exact/correct velocity number is only important for calculating trajectories etc. Having good repeatable numbers is whats important for reloading purposes. (Moral of the story, don't take the absolute number too seriously, as its probably a bit too high or a bit too low, but used correctly, most chrony's will give you consistent numbers)
HTH
AJ
