Tape a 16x24 framing square to the subject square and plumb, and snap a picture straight on minimizing parallax. Import the picture into your software. Draw a rectangle around the framing square. Map the picture into the rectangle via power clip, mask, or whatever your software calls it. Size the rectangle to 16x24. Unmap the picture. Now you have a full size image.
I layout whatever it is I'm making on this image and it's never, ever, not fit exactly as I wanted. I've been doing this for years, it takes less than a minute, and it's always been accurate to 1/8" or so.