[mistche20]

the following?:

low-spin d5 metal. Energy = -2Δ -4K (where K is exchange energy. i,e. minimization of energy due to parallel spins)

low-spin d6 metal. Energy = -1.6Δ -3K

Therefore: energy needed to add an electron = ΔE= 0.4Δ + K

Is that correct?

maybe I should be talking about Spin Pairing Energy (P) .

HEEEEEEEEEEEEEEEEEEEEEEEEEEEELP!!!!

[cheese (MSW)]

I have never seen any treatment for CFSE where exchange energy is included so ignore (small?).
For each t2g e⁻  "add" –Δo/0.4 (no eg e⁻s: + Δo/0.6)
For each t2g AO with two e⁻s (↓↑) add P the pairing energy. 
Subtract the CFSE energy for low-spin d^5
from that for low spin d^6; assume Δo is unchanged (which it is not!)
(–Δo/0.4 + P)