A low spin complex of $d^6$-cation in an octahedral field will have the following energy: ($\Delta_0$ = Crystal field splitting energy in an octahedral field, $P$ = Electron pairing energy)

In an octahedral crystal field, the $d$ orbitals split into lower energy $t_{2g}$ and higher energy $e_g$ orbitals. The energy of each electron in the $t_{2g}$ orbital is $-0.4\Delta_0$ (or $-\frac{2}{5}\Delta_0$) with respect to the barycentre. For a $d^6$ low spin complex, the strong field ligand forces all 6 electrons to pair up and occupy the lower energy $t_{2g}$ orbitals (configuration: $t_{2g}^6 e_g^0$).

The Crystal Field Splitting Energy (CFSE) is calculated as: $\text{CFSE} = 6 \times (-0.4\Delta_0) = -2.4\Delta_0 = -\frac{12}{5}\Delta_0$.

Since all 6 electrons are completely paired in the 3 available $t_{2g}$ orbitals, there are 3 electron pairs. Thus, the total pairing energy is $3P$. Therefore, the total energy of the complex is $-\frac{12}{5}\Delta_0 + 3P$.