Back when I was 12 or so, I shorted out a 1084S by sticking a metal screwdriver in that slot trying to adjust the height. I then proceeded to disassemble the monitor and broke the neck on the tube trying to squeeze it back together. That experience still haunts me to this day. The horror!
Moral of the story is to use a plastic tool to do it, or just take the back off the monitor and adjust it where it’s in plain sight. You will have access to other geometry pots on the board that are not revealed through the chassis, in case you really want to get it right.