I have been lurking around here for a while and asked some questions early on with a build I was considering building...During this conversation there were things mentioned I had not considered and have kind of put that design on the side burner for the time being...So Fast Forward now and I saw another post of a member here that had built a Concrete CNC-https://www.cnczone.com/forums/diy-cnc-router-table-machines/434442-diy-concrete-cnc.html...
Personally, this made a lot of sense, but I did not understand at the time why he did not go epoxy/granite build until later in which I learned the cost of the epoxy in relation to just how much would be needed...So, the idea of the concrete makes so much more sense...
So, while I'm trying to absorb as much as I can on this type of design and what's involved before I decide to go down this street, I have some very early on concerns I'm unable to satisfactorily overcome but I think I'm on the right path but wanted to get some other views...

Being a hobbyist and nothing in my shop/garage is flat in all directions. I do have a heavy steel shop table in which to build the base upon, however it is not dead nut flat in all directions... So how do I establish a flat foundation in which to build the frame upon and then ensure after the pour has cured that the beams in which the rails will sit upon are as flush as one can make under the circumstances...
So, what I have considered is the table is plenty big enough for the base dimensions of the frame of the CNC...
To start with get the top of the table as level as possible and if that means shimming under the various points then so be it but get it close with a level...
Then, what if I was to lay a piece of Wood/Formica down on top of the work bench and add a 1-1/2" frame to the edges of the Wood/Formica and then pour floor leveling concrete inside the area and allowed it to self-level to give me the best possible all points level foundation in which to build upon?
Again, Just tossing this 1st hurdle around of thought since I don't have access to a dead nut foundation in which is needed to lay the frame upon and then pour and cure...
Thanks
Paul