OK groupies cheap and cheerful edge finder, Mark 1, Patent pending .........TM.
For all you metrically impaired folk I'll work in inches.
Take a look in your scrap bin and sort out a sealed bearing with a OD of a nice round figure say 1". Then take a piece of silver steel or drill rod for our cousins that fits the bore of the bearing. This needs to be about 2" long. Fit the bearing with a dab of loctite and that's it, job done.
To use, chuck the drill rod shank up and start the spindle of the mill, about 200 to 600 rpm is OK, it's not critical.
Slowly bring the OD of the bearing into contact with the work, watching the bearing. As soon as the outer race makes the slightest contact with the work it will start to slow and run at about 1/2 speed. At this point raise the bearing clear of the work and you will be exactly 1/2 the bearing diameter away from the edge. This is why it's best to choose a nice figure like 1" or 3/4"
If you do have a crash and bend this - so what. Just fit a new shank.