Periodic Trends of Zeff and Electronegativity
The Slater shielding rules give more quantitative information about the amount of shielding.

Put simply, valence electrons (as you go across a row) do not shield very well, roughly 35% of a full electron. So when you go from B to C to N, you keep increasing the nuclear charge by one proton, but the electrons don't fully shield the nucleus.

Core electrons, as suggested in comments and other answers do shield fully, since they're closer to the nucleus.

So yes, the shielding for Li and Na are similar. As you mention, the valence electron in Na is further from the nucleus, and that does effect the ionization energy and electronegativity as you suggest.