Let the height of hep unit cell is 'h'. The height of octahedral voids from the base is :

Height of crystal lattice is h so effective height o h one layer be $\frac{h}{2}$. Octahedral voids is exactly between two layers so its height from base is $\left(\frac{h/2}{2}\right)=\frac{h}{4}$ and from symmetry, we can say height of second octahedral voids will be $\frac{h}{4}+\frac{h}{2}=\frac{3h}{4}$.