Can you share the latest shader code, to get more context on the problem? In C3 I would be scaling the area of the texture by 0.5, due to sprite sheets, but I though in C2, there was only one sprite/texture, so absolute values should work. With your changes to get this far, do you now have have a different scale for x,y that you need to apply to 0.5?
Also, it would be great to have an itch.io page with your effects and templates. I would be happy to buy/donate to get access.