Ordered a Chronograph and not really sure what to look for ... How to use
I found more consistent the velocity readings, better the data.
Each chronograph will have recommended distance from the muzzle to eliminate muzzle blast readings to confuse the velocity readings. My Caldwell unit recommends 10'-15' and some recommend 15'-20'. Larger the muzzle blast, further away you need to be from the muzzle for "sky windows" on top of the chronograph to get accurate readings.
And the reason why you should be towards the bottom of the "V" is to get closer to the "sky windows" for the Chronograph to measure bullet traveling over to calculate velocities. Well, that means aiming closer to the Chrono but if you are concerned, you can always place a thick metal deflection plate to protect the Chrono as my Caldwell unit has remote readout wire that comes to the bench for my smartphone. (Some will replace metal "V" rods with wooden dowels so if bullet fragments from hitting the protective metal plate shatters the wooden dowels, no problem, replace them easily).
As some posted, having consistent lighting helps with more consistent readings as I found different reading variations from morning sun, full sun, overhead sun, cloudy and dusk conditions.
And since most powders are temperature sensitive (Velocity increases as temperature increases), I also note ambient temperature for chrono data but keep in mind that it's "powder" temperature and not "ambient" temperature that matters and if your chamber is hot, chambered round will get warmer so your velocity readings can increase with subsequent shots to mess up the SD/ES numbers.
(Why doesn't anyone tell me these stuff like warm round in hot chamber will throw off higher velocity numbers if left chambered too long!
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So for more consistent readings, I keep my test rounds indoors kept at around 62F and if range conditions are too cold/hot, I will keep the rounds in the ice chest and allow sufficient warm up shots to warm the barrel/chamber then cool down to maintain more consistent barrel/chamber temperatures (Yeah, call me OCD).