You can certainly do that, and probably get a reasonable calculation of your velocity, but I guess I don't really see the point. Velocity as a number is not all that important, beyond the fact that you use it to calculate your trajectory. If you jump through your hoops to figure out your trajectory, you really don't need your velocity any more.
If you really want to know, what you could do, to get a quicker approximation is to get your elevation zero at a medium range, like 400 yards, then shoot groups at a couple of shorter distances without adjusting your sights, then play with the velocity numbers in
JBM until you get something close to matching your real world trajectory.
For example, a .308 150 Sierra SBT zeroed for elevation at 400 yards, and you shot groups that were ~9" high at 100 yards and 11 1/4" high at 200, you could work backwards to figure out your velocity was about 2800.
To get the most accurate, you need all the other variables too, sight height, temp, baro pressure, altitude.