Originally Posted by A Grade
The scale does start at 0, so once you've done 256 / 2 = 128 don't you have minus the 1 making it 127 so it fits back into the 0 to 255 scale?
What do you mean? You have an even number of bits, so you lack a specific middle level. They have to designate one as invariant for that blending mode, and it was determined to be 128. If you used either 257 or 255 total levels, you would have a distinct middle value, not that it really matters. You lose more precision to other things.