The amount (roughly) you need to move your front sight is a simple calculation. You need 6.0" of correction on the front sight.
So you multiply 6" x (sight radius measure from back of front post to front of rear leaf sight) 91/30 is about 24" so 6x24 = 144
now you need to change yardage to inches 36"x100 yds = 3600" now divide 144/3600 = .04" so you need to move your front sight .04" to the right to bring the shot group to center....roughly. I use some sort of small reference mark on the front sight....paint, small punch marks ect ect. I then use the little depth tip on my digital caliper to get to roughly where i need to be. I use a punch/hammer and block of something softer than the barrel to move my sights. tool would be easier....but my mind says thats 20+ bucks worth of ammo? same formula works for front sight change for elevation.