Chosing XTAL Capacitors for Microcontroller
From what I can tell you have some mistake in your final equation. You forgot a bracket and the two parasitic values are subtracted from each other.

Using your values it should be more like

\$ Ce = 2*(Cl - (Ci + Cs)) \$

\$ Ce = 2*(20pF - (4.71pF + 3pF)) = 24.58pF \$

So I would probably select \$ Ce=22pF \$.