The mass of product to be used in all subsequent calculations is based on the final mass of the crucible/beaker and contents (after all heatings have been completed) minus the mass of the empty container (before the magnesium was put into it). For the data listed above, the mass of product would be given by the following:

Mass of product = (46.2991 g - 45.7324 g) = 0.5667 g magnesium oxide