we take mass of alpha as helium mass,
mass of neutron as 1.00877 something in amu of course,
mass of proton as mass of hydrogen mass,
mass of atoms as the one given in periodic tables that wud be atomic.
thus masses of electrons cancel out( since hydrogen is taken instead of proton mass) and also other small errors due to electron binding energy ~ 13.6ev are negligible compared to Mevs.