This calculator will help you computes the phase matching for SHG
Negative uniaxial crystal
$\theta= arcsin[(\frac{n_c^{2w}}{n_0^w})^2 \frac{(n_0^{2w})^2 - (n_0^w)^2}{(n_0^{2w})^2 - (n_c^{2w})^2}]^{1/2}$
Postive uniaxial crystal
$\theta= arcsin[(\frac{n_c^{w}}{n_0^2w})^2 \frac{(n_0^w)^2 - (n_0^{2w})^2}{(n_0^w)^2 - (n_c^w)^2}]^{1/2}$