Looking sideways, put the ruler on the twelfth fret next to the biggest, lowest string, the one on the left (this is the sixth string). The string should be between 3/32" and a little under 1/8". Now check the same thing on the thinnest string all the way to the right (this is the first string). It should be 1/16" or a little over (technically 5/64"). If the strings on your guitar are within these measurements, or a little under, you will be fine. If they are over these measurements, though, your action is too high and should be adjusted.