Gentry-Halevi’s Fully-Homomorphic Encryption and hermite factor
It is computed like that:
L is lattice with dimension i. B is BKZ reduction of lattice L.

$$Root Hermite Factor=( B[0].norm() / ( L.volume() )^{(1/i)})^{(1/i)}$$

If you want to calculate RHF, you can use Sage Math.