Quilting Angel, depends on your ruler and how it is marked, I believe 'most' rulers all are marked in 1/8 increments.
each line on the ruler is 1/8th, on my ruler the smallest hash marks are the 1/8 lines, the medium hash marks are the 1/4 marks and the biggest hash marks are the inch mark. so, if your ruler is also marked in 1/8ths, you would be correct! 1/4 is equal to 2/8ths, so the next line is 3/8ths