This question is about emulating Gimp's Grain Merge Blending Mode in Photoshop. As can be seen, it adds the layer above and subtract 128 (DC Level).
Why is it important? It creates the ability to use "Negative Numbers" in a layer limited to the range [0, 255].
I have more than 2 layers stacked one above the other.
I want to sum them yet since the represent negative numbers (They are in the range -128 to 127) I added 128 (I can add any other number) to all.
The problem I can't add them up in Photoshop.
I saw some math tricks people made to emulate averaging like here:
Is there such trick to emulate Grain Merge in Photoshop?
P.S. The Math Behind the Blending Mode might be useful: