The answer to how you calculate VRM can be found in two parts. The first part is determining the windage, which is the resistance of the wind to the motion of the projectile. The second part is the air density, which is the density of the air that the projectile is traveling through. To calculate the VRM, you need to know the muzzle velocity, the ballistic coefficient, the windage, and the air density.
The muzzle velocity is the speed of the projectile as it exits the muzzle of the gun. The ballistic coefficient is a measure of the projectile's ability to overcome air resistance. The windage is the resistance of the wind to the motion of the projectile. The air density is the density of the air that the projectile is traveling through. To calculate the VRM, you need to know the muzzle velocity, the ballistic coefficient, the windage, and the air density.
The VRM is calculated by:
Muzzle Velocity x Ballistic Coefficient / (Windage x Air Density)
For example, if you have a muzzle velocity of 3000 ft/sec, a ballistic coefficient of 0.50, a windage of 10 mph, and an air density of 0.