This is a problem of similar triangles. The sight radius (r) and range (R) are congruent legs, and the front sight height change (h) and impact change (H) are congruent legs. All measurements must be in the same units. Therefore:

h/r = H/R or equivalently h = (Hr)/R

Example: sight radius 20 inches, range 1800 inches (50 yards), impact change 8 inches.
h = 8(20)/1800 = 0.089 inches.