Couple more things, if you have a bare muzzle, yes, get at least 15 feet off the chrono, if your using a good muzzle brake, let me rephrase that, if your using a good partition style muzzle brake, you can shoot at 10 feet or even closer with good accurate readings because the muzzle brake will redirect the muzzle blast and keep it away from the chrono.
Keeping the chrono level will give you the most accurate velocity readings, it will do nothing to change the consistancy of your velocity. If you chrono is not level, the distance between the two optical eyes will be shorter, therefore it will take less time for the bullet to cover that distance so it will tell you that your velocities are faster then they actually are. BUT it will not give you incorrect ES numbers.
A fresh battery can be a good idea also.
Shooting light can be critical with a chrono. I prefer a day with light cloud cover that still allows good light and generally speaking, shooting during the middle part of the day has been most accurate for me. Shooting early in the morning or late in the evening can produce poor shooting light and really cause strange Chrono readings. Very intense sunlight can also cause poor chrono readings as can heavy shadows.
One trick I learned a long time ago was to black out the ogive on your bullets that your chrono testing. This is generally not needed in the calibers we geneally use for long range hunting but it can help if you have a finicky chrono. I use a black sharpy and simply PAINT the exposed portion of the bullet.
I learned this trick a long time ago when trying to chrono 17 cal rounds. When velocity got over 4000 fps it was nearly impossible for the chrono to SEE these bullets. Blackened the tips and all was well.
Also happened when I was using some of the 57 gr Calhoon 22 cal bullets with their Slick Silver coating, the chrono did not like this shiny silver coating, again, painting black solved that problem.
As mentioned, generally you do not need to do this with larger calibers but if you are questioning your readings, it can help.
As far a case prep, here i what I recommend for a long range HUNTING rifle:
-Full length size virgin brass
-Trim all case mouths square and to the same length
-Evenly debur inside AND outside of the case mouths
-Seat primers with even pressure
-Measure powder charges to +/- 0.1 grain
-Seat bullets to within +/- 0.001" in length measuring Ogive to land clearance, not OAL.
Additional things you can do if you want to get into it. In my opinion, these next steps WILL improve your consistancy but for shooting in field conditions they will not offer the returns as high as the above items. The one exception would be for EXTREME range shooting, that being over 1K yards.
-Sort cases by case volume (not by case weight)
-Sort bullets by weight
-Sort bullets by baring surface length
-Neck turning (take very light passes to even up neck thickness)
-Square primer pockets and cut to uniform depth
In my testing the above list will get you the most bang for your buck for a long range hunting rifle, the second list is for those that like bench work or will just sleep much better then if they did not do these steps. I sleep just fine not doing them though