All rifles are individuals so you only need to be concerned with the velocity of your loads from your barrel. How much faster the velocity is from a longer barrel is of no consequence, unless you have started with a long barrel and intend to shorten it. The velocity you observe after shooting across a chronograph might be within the range of what is expected or it could be a little higher or a little lower. In a hunting rifle this doesn't matter. What does matter is if the first cold barrel shot always prints to the same place, and that follow-up shots print close enough to be considered a group. If your rifle doesn't do this you need a different load or the rifle requires attention. You must still choose the correct bullet for the job at hand and you must limit your shots on game to the ranges at which you know you have a reasonable chance of success.
Handloading will give you a better understanding of how different components effect the performance of your rifle and will encourage you to shoot more. Shooting more and keeping accurate records over time provides a valuable reference.
Shooting is the best way of determining your trajectory, and provides the most accurate data you need to effectively correct for range and wind. The flattest portion of the trajectory is from the muzzle to your zero range, after which the bullet dives off quickly. A 200 yard zero is not unreasonable for a .308, and will require a zero which could result in the bullet hitting 2.5" high at 100 yards.
