Maybe you measured hole distance with vernier calipers.

It's not exact cause hard to center its teeth.

Simple 3D printed parts can handle it.

Step 1: Hole Measuring Tool

It's like vernier calipers but its teeth is cone shape.

So cone's center must be hole's center

and can measure center to center.

Step 2: Design It

Draw cone and cut slit. The slit's width is your venier calipers's thickness.

Calipers' tooth should be center of cone. cut radius deep.

Now print. Mine are not peaky. but now just go on.

I made a bolt hole to fix but didn't need a bolt to fix it

And two cone's distance should be 20mm, but it's 21mm

Instead re-printing it I will minus 1mm.

Now you can measure hole distance easily.

You can measure side to hole with one cone.