Lost in the dark

Still wandering around trying to get all the lighting exactly right.  This is taking more time than I expected.

I have a formula for illuminating from a single source (the player) but I'll eventually want to have other light sources in the dungeon.  Old fire pits, torches, lava, etc.  This sounded like an easy add, but the question quickly came up "What do you do when two light sources touch the same tile?"  The quick answer was "add their light values together".   The problem arises when you have to turn off one of those sources.  How much should you take out of the other tile?  For movement I had been wiping the player's torch completely and adding it back in, but that might not be the right answer anymore.

At least I figured out a universal lighting formula that I can use for every tile in the light radius without having to adjust positives and negatives manually.