Just use the stock one. When and me were working together we used some offline renderer (Mitsuba Renderer) as reference and it showed very similar results. The only thing you may want to tweak is the option I mentioned at #222 if you have tons of memory and want to make it faster. Basically when bUseFilteredCubemapForSkylight is turned off it doesn’t mean using a non-filtered cubemap, but not using a cubemap at all (it will use a super blurry SH representation of skylight which loses most of directionality and is not physically correct). So unless you want to intentionally make the environment lighting more ‘averaged out’, you probably never need that option.