Explosion render elements for illum/gi shdw on ground

[ttdww]

Seems like this is something I've dealt with (faked) for years, however was wondering if there is a mathematically correct combination of render elements to rebuild a comp of blast-beauty along with fire illumination on ground, but occluded by gi shadow (also hued by fire light where accurate), and with shadow occluding the ground of course. Attached is what I'll call a faked method where I have beauty over shadow element over ground plate, with illumination element setup as just fire light illuminating a white vray plane (with scene lights turned off), and then 'added' between beauty and shadow. I'm presuming this isn't really accurate though and will cause issues in some cases. Green arrows point to where seems wrong. Is there a better way? If there isn't a 'correct' way, I'll settle for any user tips to cheat more accurately ;D Thanks!