Yes, i meant in mm. But to tell you the truth i found that the hardest thing is to avoid for dust not to get trapped in between when mounting, cause that could f%%ck all previously achieved precision. When i lay the rails i lay the edge first and scrap them against the epoxy so no dust goes trapped there. And there is a lot of particles of it even in a new workshop :-)

My straight edge was perfectly flat against he epoxy when i was happy with the result. But even in my best result, at one end of the bed under one of the rails there was need to shim / if you read the whole thread about my machine you will remember that i was experimenting a lot with the epoxy pour, so i found the way to do it properly/

So yes, to your idea about the DIY epoxy straight edge, but remember the main thing i discovered when pouring 3m rails, i needed to over extend the epoxy both sides 25cm/?? read the thread again but make it so to be on the safe side/ so it shrinks there, after that just cut it to the size of the profile.Similar to what you will do with the meniscus.