Great Question! I have been told Trajectory Validation. But I have not adjusted it as of yet. I am on to 750 and in so far. I will be shooting 1000 this weekend and would like to hear this from someone that has adjusted it first hand both ways.
I've been using exbal for about 2 seasons now and have shot to 1200 using the standard b.c. figures but I don't trust it. I can shoot 700 all day long. But I would like to extend my range with confidence. I have a nice chrono but I can't get myself to trust it after having so many pieces of junk.
Maybe I'm just lucky, but last year we verified the drops of my 338AM out to 1515yds, exbal was perfect with just a change in the BC. Kirby had told me that .8 would be real close for the 300SMK. We loaded all the weather data etc into the palm and it was perfect.
Who knows, maybe I should have bought a lotto ticket that day?
So my experience is to adjust the BC to fit your drops, (assuming you have a load that has consistent velocities).
According to Exbal, my 338AM at 3300fps and 3250fps will have a difference of 6" at 1000yds. I think most folks strive for single digit ES's and typically will settle for ES's less than 20 for the big cartridges.
On the other end of the power spectrum, according to Exbal. A .30cal 168gr Berger VLD will vary 17" at 1000yds from 2600fps to 2550fps.
So you'd want a much smaller ES for more ballistically challenged chamberings. <-- Politically correct terminology for the .308 Win