Bought a used SP101 and after a couple trips to range off sandbag at 12 yds found it consistently shoots 3 inches left. Elevation is dead on with 158 grain 357 and varies with different grain and brand 357's as well as 38 special loads but all are left about 3 inches. Looking at filing front or rear sights a little and was wondering if any math folks out there could give me some advice on approx. how much to take off as a minimum. Don't want to take to a smith without trying to adjust the fixed sights myself first.Thanks for any suggestions