You have to have two very accurate chronographs. I Did this with two Beta chronies ended up with a range of values from .687 to 1.3 for the LRB 120gr J36. This was only at 100 yds. though. Perhaps at 300 yds., there would be enough difference in the velocities to downplay the intrinsic error in the chronies.
Check both chronys to make sure that they are dead on... set them both up at 20 or so feet, and do as Dantec said... shoot a consistant match grade .22 rf through them... 20 rounds each to get a accurate average, before you go any further.
Also, remember that the close chrono is five to ten feet in front of the muzzle, you need to have 100 yds (or better 200 to 300 yds) between the center of the chronos, not from the muzzle, for an accurate velocity lose calculation.
Depending on your software, you might have to measure the velocity at several distances, because the software only recognizes G1 form factors (like Sierra "Infinity"), and these are not accurate with just two readings (especially with 100 yds separations).
If you are shooting the LRBT bullets, how are you finding their accuracy... they are touted as the most accurate bullet in the world, but I have yet to find anyone that has had that kind of accuracy.
If you have (or can borrow) the 4th eddition of the Sierra loading manuals, there is a lengthy discourse on why their BS have changed values (mostly DOWN) over the years. They explain the problems they had with inacurate instruments, errors in trap spacing, etc... it is not the easiest thing to do with accuracy (as you have already seen), as there are lots of gremlins waiting to bite you in the butt.