I read through the manual and find nothing really about how cloud thickness works.  Is the higher the number, less visibility or vice versa?

Cloud thickness is, as the name suggests, the thickness of the clouds.

If the cloud base is 2,000ft and thickness is 5,000ft then the top of the cloud is 7,000ft.

 

The 1/8 - 8/8 number is the amount of cloud cover as a percentage and is the standard way of expressing cloud cover, 1/8 oktas is 'Few' and the numbers range through 'scattered' to overcast which is 8/8 oktas.

 

Hope that helps.