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

I understand that the at. mass unit is based on 1/12 the mass of a C-12 atom.  C-
12 has 6 p, 6 n, and 6 e.

What I can't find information on is how the proton is determined to have a rest 
mass of 1.0078 u and a neutron of 1.0087 u.

Can you explain this?

[Below is mass of C-12 atom, using amu. Is this significant to the question?]
1.0078 x 6 p = 6.0468 u
1.0087 x 6 n = 6.522 u
0.00054 x 6 e = 0.0033
Mass of the C-12 atom = 12.10 u

Thanks.

DF

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