RE: Chronograph ?
Cloudy days are best, followed by sunny days with the diffusers on the screens. Try to shoot through the middle of the screens as much as possible. Another thing you might want to try is moving the chrony out to at least 10 ft from the muzzle. if you're too close to the chrony the gasses from the muzzle can actually disrupt the reading. Sometime this will cause a wild reading, but since it usually effects the first screen, the chrony will just show no reading at all because the first screen gets tripped twice before the second screen gets tripped. This messes up the timer.
As far as published vs. actual muzzle velocity, well, you're in for some surprises, some good some bad. One thing to note in your manual is what type of barreled action the round was pressure tested in for the published data. Sometimes they use pressure test barrels, sometimes they use factory sporting rifles, sometimes they use longer than typical custom rifles. One thing you'll likely note is that velocities taken from pressure barrels are considerably higher than standard because they use minimum spec chambers and throats with tight headspacing. This tends to give higher velocities. The data from custom rifles will probably be a little higher than what you might see in your rifle because custom guns usually have the chambers tighter than factory rifles, but not min spec. And if you're lucky and the data you're using was shot in a factory rifle with the same barrel length, it'll probably be pretty close, with your rifle maybe even exceeding published velocity. Factory chambers are usually cut on the loose side so that they will reliably and safely feed, chamber and fire all factory ammo. Pressure barrels are intentionally min spec because if the pressure is safe in the test barrel, it'll almost certainly be safe in your rifles barrel.
Mike