In my opinion Criss's tip on drilling a hole is the best, lowest risk path. Using the excellent pictures here, I scaled the image to reality and find that the adjusting screw centre is 10.9mm up from the bottom aluminium edge and 20.8mm in the left hand edge. I stuck a sliver of masking tape on the plastic and drew a +. As well as allowing a pencil to be used, the tape stops the drill head skating. I finger drilled with a 3mm bit and the T6 socket appeared in the dead centre of the hole.
In my case, both left and right clicks were sluggish so I swung the screw a full turn clockwise and the same anticlockwise. I then stepped through to find the best clicking setting.
3mm is a very neat hole but I have stuck a sliver of scotch tape over to keep bugs & Sahara sand dust out (I live in the S of France). Job done. Thanks everyone!