Following situation I need to align my linear x rail. The rail is 4000mm long an 20mm wide. So I couldn't afford to mill an edge to lean it on. So I decided to make an epoxy bed 40mm wide and 5mm high to level it in z. Now I'm thinking of how to get it straight. I have a straight edge wich is class 0 so it's pretty straight. But it's only 500mm long. I found in the forum a clever method by using an electric wire.
https://www.cnczone.com/forums/mechanical-calculations-engineering-design/19418-layout-centerlines.html
But it's from 2006. So I wonder if there are some more recent ideas to solve it. Maybe you could direct me there