Re: Exactly how is the 1.0078 u mass of the proton determined?

 The  proton mass should be 1.0073 u.  This makes the mass of 6 protons, 
neutrons, and electrons add up to 12.099 u.  The 0.099 u difference 
between 6p+6n+6e and C-12 corresponds to (.099 u X 931.49 MeV/u =) 92 MeV 
which is the binding energy of C-12.
The mass of a charged particle like the proton or ionized C-12 is usually 
determined by measuring its momentum (P) and energy (E), and then using 
the Special Relativity relation m^2=E^2-P^2 (in units where c=1).  
The mass of the neutron is determined from the maximum energy of the 
electrons emitted in the beta decay of the neutron to a proton, electron, 
and anti-neutrino.