Using Milescraft pocket hole jig

Hi,
I have a Milescraft 1325-PcketJig2000 for pocket holes and I cannot figure out how to set the thickness if it's not one of the pre-set thicknesses.
Here is the link to the product and the manual. It seems to imply that I can use wood of any thickness and it has some common thicknesses preset, such as 12,19,27 and 38mm. But if I try and set it at anything else, the little locking mechanism stays proud of the surface. This means I can't place it flat against the wood.

For example, if I have a piece of wood that is 16mm I can't seem to use this jig. I think I'm doing something wrong. Do I just need to select the next best width? So for a 16mm I would select 19mm? It seems like something that they would mention in the instructions, but there isn't anything.

I've included a photo of what it looks like.

Shared album - Kobe James - Google Photos

Any help would be appreciated.

Thank you