 Single-scattering forward-plus-isotropic flux partition

If fraction $a$ of scattered energy is exactly forward and the rest isotropic, a single-escape slab has upward fraction $(1-a)(1-t)/2$ and downward fraction $t+[a+(1-a)/2](1-t)$. They sum to one for conservative scattering; this approximation need not be accurate for a thick cloud.