Computers/software might not know what the scanned DPI was, so you might have an image that was scanned at 150 dpi, but when printed the software may assume that it was 72 or 300 dpi. Modern software is usually smart enough to handle this correctly, but not always.
When you print the image of the ruler, measure the printed 1" on the paper against an actual ruler's 1" length. If they print equal, then you are good. If you measure the image's 1" to be different on the printed paper, like 1.25" measured with an actual ruler, for example, then you would need to print the image at a percentage to get the correct size, 1-image to 1.25-actual, or 0.8, i.e. 80% in this example. You can also scale the image in some image manipulation software like GIMP or Inkscape to the correct size with the reference of the ruler in the image.
