Doug, The first two frames(C 1980) I built were Proteus frame kits (book included). Came with all the tubes mitered. Pretty easy to build since the hard work was done and I had access to good tools. When I built the MTB I used a tube notcher from work. The notcher's holesaw was not well suited for thin wall tubing and tended to pull to one side of the tube. I forget who made the stamped lugs but they were really sloppy and didn't offer much of a reference point. I no longer had access to a head tube facer and reamer so I used a flat file and Dremel. All that coupled with my lack of knowledge, That 12mm of twist could have been worse.
Once I had a plate to reference from,I got some practical advice on procedure and paid more attention to details on the next MTB frame . 25 years later I still ride it most everyday.
My plate is 25"x 34", 45"+ long would be better so I could more easily/accurately check the alignment of an assembled frame and fork. For a surface gauge I use a dial indicator on a magnetic base. Since I only make a frame every 3-6 years my surface plate spends the majority of it's time as a jig for gluing up cabinet doors.