A quick hint: To file something down perfectly flat, you require (a) a flat file (flat both sides), and (b) a worksurface. Remove handle of file, place file on surface, and rub item to be filed across file instead of the other way round. This will at least be flat in the direction you're filing, although it might be rounded slightly in the other direction, but even then it should be better assuming your hands are fairly steady.

Graham.