I usually test DMM's on 4 DC voltages, 2.5, 5.0, 7.5, and 10. Based on my experience with the many BM257's I've tested and sold,
they are almost always spot on or within 1 count on 2.5, 7.5 and 10V. It is usually about 2-5 counts (3rd decimal place) high on the 5V, but even that's only 0.04-0.1% out, which is still well within its specs (0.2%+3d).