Using a webcam or microscope to visually set the reference point on a CNC, is very comfortable. Unfortunately, due to the short focal length, these devices need to be very close to the workpiece.

A way to keep a safe distance is by adding a Telephoto lens.

Here you have a very simple adapter to plug a microscope and a tele lens, both available on Ebay.

I also designed a simple (but flimsy, needs improvement) support for a CNC mill.

The result is fantastic! It works!. The only problem is centering the optical axis, since the microscope's sensor is slightly tilted. This causes a slight displacement of the crosshairs over the workpiece as you raise or lower the Z axis. But it can be corrected with a little tweaking.

Designed with Cubify Design (Alibre).
Source files also posted.

Crosshairs photo: distance between ruler marks is 0.5mm.

The wall mount is designed to hold the optics with zip-ties.
It uses as little material as possible.
I'll design a fully protecting enclosure in the future.