You are correct that the tick marks really should be 1/8" apart. Do the related questions ask the student to identify a specific point on the plot? I would be inclined to show only the ticks in increments of 5. I usually import the .jpeg into my graphics program and trace, so the placement would be the same as in print, and the student could still roughly calculate the position of the points. I think this option is still far better than trying to split the number line. Another alternative would be to keep the horizontal position and create a fold-out page.