Well, at least you didn't say:
The first thing you have to do is to calculate the tangent of Alpha, Alpha being the angle from the line of sight to the hypotenuse of the triangle formed by the actual location of the impact of the projectile.
With the impact being 6 inches from the line of sight at a 25 yard distance, following converting 25 yards to inches we have:
6/900 = Tangent Alpha = .0067, therefore determining Tan ^-1 Alpha, we find that Alpha = 0.382 degrees.
Now measure the distance between the sights and multiply that value times Tan 0.382 degrees.
If the distance between the sights is 10 inches, we have
10 inches X (Tan 0.382 degrees) = 10 X (.0067) = .067 inches. :grin:
(See? Trigonometry isn't that difficult, is it?) :rotf: