Widening the rear sights is a common mod. The most popular width is right around .140. If you are going to widen the rear sight the first thing to do is shoot the gun and see if the windage is off. There are some formulas for correcting windage. So, for example, if you determine that to get the windage correct you need to move .010" to the right when you mill or file the frame make the cut .010" to the right of the center of the original sight groove.