Here is how I build a centerline finder for neck set and bridge positioning:

The Construction I made in Autodesc Fusion 360


The tool path for milling I also made with fusion 360:

The first part I have build is the ruler. This part is milled out of transparent acryl:

Beside centering the neck I decided to add a scale to check an realign the bridge if needed:

The centering will be done by four bolts, that are sliding in four angled slots. Each pair of this bolts are pushed by a t-glide in a center slot through links. One of these links on a pair has a washer, so it’s laying parallel to the surface of the ruler:

The T-Glides I made out of plywood. I milled a pair out of a 20 mm thick peace.

The bolts and the nuts I turned out of a round peace of beech wood.

The first Test:

The Build on YouTube: