As a rough guide, insert thirds between f:8 and f:11. 8 +1/3 stop = f:9, 8 + 2/3 stop = f:10.

4.5, 6.3, 9, 13, 18, 25, 36, 50, 72, 100 and so on should be familiar to all who use old German lenses - especially as max aperture!

5, 7, 10, 14, 20, 28 and so on is less familiar, but possible on old French lenses.