Measuring Electronegativity

The general chemistry textbook I'm reading explains how Linus Pauling came up with his electronegativty scale. Given a pair of atoms A and B, if we let ΔE_AA and ΔE_BB be the dissociation energies of an A-A and a B-B bond respectively, then an estimate of the covalent component of the bond energy of an A-B bond is (ΔE_AA*ΔE_BB^1/2. So that if ΔE_AB is the dissociation energy of an A-B bond, then

Δ= ΔE_AB - (ΔE_AA*ΔE_BB^1/2

is a measure of the ionic component of the bond. More specifically, Pauling defined

X_A - X_B = 0.102Δ^1/2

Where X_A and X_B are the electronegativities of atoms A and B and Δ is the expression in the prior equation.

I have some questions. If I can directly observe in the lab the dissociation energies of an A-A bond, a B-B bond and a A-B bond, then I can compute the difference between the electronegativities of A and B, but how would I arrive at the exact number listed on the periodic table if all I can observe are differences? Would I have to set one element to 1 and then set all electronegativities relative to that one? Also, wouldn't the subtle influences of the rest of the molecule cause these bonds to have slightly varying dissociation energies depending on the molecule they're in? Which one is the "real" dissociation energy? And finally, where did the 0.102 come out of? It seem kind of ad hoc to me and my book doesn't explain it further.

As far as I remember, Pauling originally used arithmetic mean value, not gemetrical one (as in your post). However, results are almost the same .
Google will tell you the rest ( if you bother yourself using it).