[CAD_CAM_EDM_DRO] Which to trust, home-built DROs or hash marks?

I think that I can add a small verse that may answer your question:

When you go out to sea
take one clock or three.

This is an old english sailor's saying afaik. And it is true. Had you had
one dial to read you would have trusted it. Having two different ones, you
are confused, having three different ones is even worse.

The right thing to do would be to obtain a caliper caliber (I do not know
what it's called in your parts). Usually it's a cube of metal machined
very accurately to 1x1x1 inches or similar, that comes in a small padded
box. That would allow you to check your calipers. It may be shaped as a
square ruler or triangular section ruler or cube. I do not know where to
get one now but these must exist. They used to just be there in machine
shops. Probably NIST can tell you where to get one in your country.

Peter