With hunting season coming up, I hope this can help some of us. I just went through this with my .36 cal flintlock with fixed sights, but it can be used with any open sighted rifle. Fire at 25 yards, use as small a target as you can to get the smallest groups you can. Aim at the bull every time, no "kentucky windage". Measure from the center of your group to the center of the bull. You need two measurements, one windage, one elevation. Measure at 90 degrees. Ex: one inch right, two inches high. One number at a time, divide error by 900. Ex: One inch right equals 1/900, two inches high equals 2/900. Windage now equals A, elevation now equals B. Measure your sight radius, the distance between your front and rear sight. This equals C. A multiplied by C gives you measurement in inches how far to drift your front sight to the right (if missig to the right, drift left if missing to the left). B multiplied by C gives you a measurement in inches to either lower (file down) your front sight or raise your rear sight (assuming a high miss, opposite if low) The numbers given are for twenty five yards. If shooting at fifty yards, use the number 1,800 rather than 900. If shooting at one hundred yards, use the number 3,600 rather than 900. Zeroing my flintlock, I was dead level, but 4 inches to the left. 4/900=.00444. .00444X36=.15984. So, I drift my sights .160" to the left, bringing my POI to the right Hope this helps all of you and takes some of the guesswork out of iron sight changes.