Buy high-quality silver wire from a goldsmith, it comes in exceptional purity (meaning that the density is right at the 107.9 g/mol spot), and the wire is also always of a certain known diameter. As you know the density and the diameter (and hence the "bottom area") of the thread, it is easy to calculate how long bits of it you should cut to achieve 1, 2, 5, 10 and 25mg weights for your calibration.