Good question. I reasoned that profile rails and supported rails conform to whatever you bolt them to, but round rails have their own built in straightness. By bolting the X and Y blocks either side of the same plate, I think I have defined a flat XY plane and I can grind the table square to that.

The tricky bit is cutting the inside of the RSJ channel flat and square where the rails connect. There is no sensible reference surface anywhere, tolerance seems to be about 1.5mm. I am hoping for consistency.

I got a wonderful gadget from RDG Tools, a sort of spring loaded centre. This held the m30 reamer amazingly square and true when finishing the end supports. I can slide all 6 onto the same pair of rails and they happily slide about and butt up against each other. I was not expecting that, good tool.