The part in red just seems to have a  misunderstanding of how math works, and I did answer the green part with  10/3 inches.
Here's something to try, and I'm not sure if this will work because I'm  not sure how exacting photoshop software is (and I don't have it  anymore).  Create a 12 inch ruler or rectangle in photoshop, cropped so  that the available space is exactly 12 inches across, and line it up  with their ruler lines, then make a mark at 4 inches, and at 8 inches.   Next, scale the whole image so that it is ten inches wide, and if  photoshop is really exact, it should scale all the lines accordingly,  and the marks you made will split the now ten inch ruler into three  equal parts.