Brownell's used to sell a proper file designed specifically for this. I think the file Brownell's had cuts a 140 degree angle. It is not an angle that was available on files sold in hardware stores.

I did two completely homemade rear sights several years ago using the following method once at the step where it was necessary to cut the notch:

I used a 135 degree v shaped file, which should be available in your local area.

Make certain there is a guide notch in the exact center of the top of the rear blade, if not carefully make one before starting. If possible to get a rear sight that already has the v notch partially done, that would be a better choice, because it is very easy to start the notch with misalignment and if off by even a degree or so it can look abysmal when done. Dugaboy's method of marking sounds a lot better than what I did.

When using the file to cut the v notch, the file stroke should be moved FORWARD ONLY toward the muzzle to insure a clean edge on the rear of the sight.

If you work the file both directions, the the top edge of the v notch in the sight will be rounded, making for a sloppy sight picture when in use. When getting close to point of aim, test fire it after every couple of strokes of the file, because the permanent adjustment will pass through point of aim very quickly.

Remember to practice on scrap metal first.

Richard.