Zeff and ionization energies

orgohell

Hey guys, Im having trouble understanding/explaining some of the trends in ionization energy with calculated Zeff by slaters rules.
For example I was asked why is the 3rd ionization energy for Be greater then the 3rd ionization energy for O


My train of thought was to say its pulling an electron from a complete shell in Be (1s²) vs. pulling an electron from the 2p orbital....but how do I explain this with Zeff?

-Zeff of the 1s electron of Be²⁺ is=4-(1 x 0.35)=3.65
-Zeff of the 2p electron of O²⁺ is=8-(2 x .85)-(3 x 0.35)=5.25

so the p electron in the O still has a higher Zeff and wouldnt that mean its held tighter to the nucleus and more difficult to remove?

What am I missing here?