How do you guys keep track of pixels in upscaled paintings?!

I have a source image that is 69x38 pixels. I'm painting it at 2x (2x2 = source pixel if brain fart), and I keep losing track of where or the scale going from source to map.
I've seen many huge paintings (which I know is lots of collaboration), but I wonder how one keeps everything accurate?
Am I just bad at this? haha..