How to Calculate Windage & Elevation
When shooting a rifle, windage measures the horizontal distance of the impact point from the target, and elevation measures the vertical distance of the impact point from the target. Finding the correct windage and elevation adjustments for a given distance when sighting in a rifle is extremely important for precision shooting. You can use a formula to manually calculate windage and elevation measurements to properly adjust rifle sights.
Things You'll Need
 Rifle with milliradian (mil) markers in the scope

Identify an object of known size in inches. Look at the object through the scope on your rifle and measure the object's size in mils. Enter the measurements into the formula below:
I x 27.8 / M = Distance in yards
Replace “I” in the formula with the size of the object in inches and replace the “M” in the formula with the size of the object in mils. The formula result is the distance to the target in yards.

Aim at the target and fire three rounds. Look through the scope and note the average distance in inches that the impact point is away from the intended impact point.

Find the MOA (minute of angle) adjustment needed to correct the impact point using the formula below:
Inches / Distance = MOAs
Replace “Inches” in the formula with the windage in inches that the impact point must move to hit the intended impact point. Replace “Distance” in the formula with the distance to the target in yards. The resultant answer is the number of MOAs the scope must be adjusted to correct the windage impact point.

Repeat Step 3, except replace “Inches” in the formula with the elevation in inches that the impact point must move to hit the intended impact point. The resultant answer is the number of MOAs the scope must be adjusted to correct the elevation impact point.
References
 Photo Credit sniper rifle on the tripod and optical sight image by Vladimir Melnik from Fotolia.com