If you are going off the velocity on the box chances are you are way off. Even most optical chronographs suck.
Find someone with either a labradar or a magnetospeed. Even then, chances are your velocity will be affected by your powder temperature. That variable is determined by which powder is used with that load. Some powders are much more temp sensitive than others.
Outside of that, external temp, altitude, barometric pressure, etc can change things a great deal.
Again, 500 yards and in on deer sized game or larger these variables are minimal enough to make pre marked dials like the CDS an okay system with most calibers.
Push beyond that you are asking for issues. You will end up with wounded or missed game without a solid understanding of how these variables affect things and how to compensate/calculate for them.