I'm confused, are you saying you would want the blend to be lighter then light (255,255,255), or do you want it to wrap around back to black (0,0,0) and add up the leftover values from there?
I think he means the latter, but if so, his maths is wrong. It would actually wrap to 65,65,65. Or maybe he means it starts counting backwards from 255,255,255 back to zero.
I don't know of a blend mode that does either of those though.
You could do what Genexi suggests, but that's gonna be slow doing pixel operations for blending.
Could you possibly describe the visual effect you're trying to achieve? Perhaps there's another way or a similar but not quite identical blend mode.