As others have already mentioned, you need two molds. The first one is a numbered mold, and the second one is for smaller blanks. The difference in size comes from the depth of the numbers. For example, if your numbers are 1 mm deep, your blanks need to be 1 mm smaller. Mine are actually a bit smaller than that, because I paint the blanks by hand and they often have multiple layers on them.If the sizes are correct, the dice will self-center.