How to make a pixelate post process material.

I want to create a pixelate post process material that has a constant resolution independant of the actual screen screen resolution.

https://forums.unrealengine.com/community/community-content-tools-and-tutorials/40692-tutorial-8-bit-post-process-material?69570-Tutorial-8-Bit-Post-Process-Material=

That’s resolution dependant. I want a constant resolution. For example, an iphone has a much lower resolution than a high end gaming computer, but I want them to have the exact same simulated resolution.