Octahedral Crystal Field Splitting Orbital Energy Levels
The total energy of the d orbitals before and after the application of the octahedral (cubic) field has to be the same. As three levels fall and two rise, the ratio is as shown. The $\Delta_0$ arises from the solution of the equations. (In older literature the total gap is $10Dq$ with the levels $6Dq$ and $-4Dq$.) 

If $V_0$ is the interaction energy $3\Delta_0/5=\int\psi(e_g)^*V_0\psi(e_g)d\tau$ and $-2\Delta_0/5=\int\psi(t_{2g})^*V_0\psi(t_{2g})d\tau$. In practice $\Delta_0$ is found empirically. 

The order of levels is reversed in a tetrahedral field and the size of $\Delta_0$ is also changed to $-9/4\Delta_0$.