Assume your ammo does not change, then the following calculation will work:
Draw a right angle triangle with base 100m and height 8 inches. That is the same in cm as10000cm long and 20cm high. The ratio of base:height is 500:1. Measure the distance of your rear sight to your sight blade. Divide this distance by 500. Increase the height of your front sight by this amount. Your point of impact will now be 8 inches lower.
Rob