If you draw a line from bullet hole to bullet hole forming a triangle and then put a dot or an x or whatever centered in the triangle, that center point will be similar on targets 3,4, and 5. Therefore with 4 being the middle of the 3 loads, it would be your ocw start point. Then you would begin tightening up the groups by adjusting cartridge overall load length. You didn't specify your load to load increment, but could also adjust a 1/10 grain or so either side of target 4's charge to whittle group down possibly even further.