I just looked up the hiwin specs, they say 130um over 500mm max allowable surface height error between two rails in the same plane, so I think this equates to an angle of 0.015 degrees or 53 arcseconds, I would be around 4x that with the rails mounted directly to the first slot. It would probably be absolutely fine but anyway.

I've made up some feet from spare plate. These are bolted into the bottom of the extrusion and will then be clamped to my milling table.

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I've just trammed the head to the table, and checked the table over the entire X travel. The table is within 0.01mm, i.e. I can't measure any error with a 0.01mm resolution DTI. Think i'm just going to flycut the top and bottom of the entire extrusion, then I'll have the option of mounting to a slot or tapping into the centre. No time to do it today taking the kids to see the new star wars film this afternoon.