To fit a rear sight to a standard 3/8" dovetail, always stone the bottom flat of the rear sight and never the edges.
Never use a file either.
To properly stone the flat use a good bench stone of medium cut, oil well and pass the bottom flat down the full length of the stone and trial fit.
The sight should just start into the dovetail without resistence.
If more passes are need to gain proper fit, always turn the sight 90 degrees for each successive pass down the stone.
never use a steel punch to seat the sight into the dovetail either.
Always use a brass or better yet, delrin punch to achieve the most professional results.
A proper sight pusher is even better. HTH