I should have added,
With a front sight about 0.110" wide, I use a rear sight with a slit of about 0.040". I have no files that thin. The aim, the front sight is level across the top of the rear. The slit of light in either side of the front sight of about the right width with those dimensions on a long rifle. The slightest misalignment is obvious. Even when using a tang sight not perfectly centering the peep hole is obvious in the rear sight. The wide front sight is a good width for the 50 yard aggregate target black.
For field shooting or trail walks it may not be optimal. It does make the smallest groups on paper targets for me.