JCS description of 3 plate method using glass and bondo beddding on ply comes from the Gingery books. Gingery later concluded that the bondo and ply shrank and threw the plate off in flatness.
Pekka I think you misunderstand me. The ground carpenter's level is to be used as a straightedge, not a level, and it (or any other straightedge) would need to be longer than yourf milling table -- say 1200 mm, minimum. I believe that such a level can be found at very reasonable price ground to less than a .001" overall, which I believe fits your rquested precision.
You do not necessarily need a surface plate for a limited tolerance on your milling table. You can test with a 1200 mm straightedge laid lengthwise at front and back and crosswise, as well as diagonally. Even a conical (twisted) straight solid will register as nonlinear if the straightedge is not placed directly on a radial.
When i built my Gingery lathe bed I used a 4 foot long ground carpenter's level, with a guaranteed .001" edge straightness tolerance over that length. I believe it cost about $40 US at he time.
You can also make a straightedge using the method of threes from hot rolled steel stock -- say 1/2" by 2" by 4' (or similar metric).
The method of threes is simple:
Grind with lapping compound any two surfaces (called say A and B) until they conform. This may not be straight, but they will be mirror images. Then lap a third surface (let's call it C) to A.
It will now be similar to B.
Then lap C and B together. Since they will have relatively corresponding high and low areas, the lapping will produce a reasonably flat surface. This method can be repeated if extremes of accuracy are required, but generally one go is sufficient for home machinist's needs.
I made a straightedge for the new lathe I'm building, but used another method of comparing it to a surface plate I already had, and scraping.
Scraping or lapping a straightedge by the method of threes would be much easier and quicker than doing the same with 1200 mm square surface plates, and because of the low mass, a straightedge needs far less stiffening, for your limited accuracy puroposes.