Back on stage,
I noticed along the past few months that there was a kind of contest about making a lathe tool height gauge.
So here is my proposition :
It is simply made with a round piece of mild steel, with a groove precisely located so that the top of the groove is exactly at the center height.
The special feature I added is a neodymium magnet screwed at the bottom end, which provides a solid bond to the cross slide surface, still allowing for an easy sliding lateral motion :
Should have done that earlier ...