Although eqn. ( ) takes a simpler functional form than eqn. (11), both are likely to involve similar computational effort to evaluate, due to the fact that the spherical harmonics Y lm (r) of eqn. (12) and the Legendre polynomials P l (cos θ j ) of eqn. ( ) are homogeneous polynomials of degree l with 2l + 1 components; the summation over l in eqn. (11) therefore has O(L 2 ) terms, similar to the correspsonding Lth degree t-design. Using this duality, we adopt the method described in ref. 21, particularly eqns. ( ) to (18), to estimate the required cubature parameters N GH and M . Pruning the grid by choosing a different t-design for each Gauss-Hermite quadrature point is one potential route to safely reducing the number of points to process.