You can determine the mass defect when you calculate for the absolute difference of the experimentally determined molar mass and the theoretical molar mass.
64Ni means there are 28 protons, 28 electrons and (64 - 28) = 36 neutrons. The mass of each proton is 1.0072766 amu; the mass of each electron is 0.00054 amu; the mass of each neutron is 1.008664 amu.
Theoretical molar mass = 28(1.0072766 amu) + 28(0.00054 amu) + 36(1.008664) = 64.5308 amu
Thus,
Mass defect = |63.9280 amu - 64.5308 amu| = 0.6028 amu
