<p>Between the markings in the 100s it is probably 1/3 stops.</p>
<p>e.g. 100, 125, 160, 200, 250, 320, 400, 500, 640, 800.<br>
64 is 2/3 stops down from 100 so there might be one mark between 64 and 100 and 32 is a whole stop from 64 so there are probably two 1/3 stop markings between them.</p>