After watching Youtube vids from Joe Pieczynski https://www.youtube.com/watch?v=1MrjnIcscxI and Rober Witkamp https://www.youtube.com/watch?v=iNhQYFsou1Y, I decided I had better make a lath tool height gauge for my Colchester 1800.
The gauge is made from stainless steel at the top and aluminium at the bottom, with a 20mm x 10mm rare-earth magnet epoxied into the base, inset about 0.2mm. I used a hydraulic press to fit a spigot turned on the stainless section into a reamed hole in the aluminium. Didn’t feel the need for a roll pin or thread.
Overall height is about 160mm, diameter is 30mm. Using a fingernail, I can now set my tools to within a few tens of micrometres of centre in a few seconds.