Hello, cozmicray.
Unless you are building a lab-grade machine (which would exceed the resolution of most 3D printing methods), don't worry so much about static surface flatness. The 0.5 AA specification is a pretty tight tolerance for flatness, but would be susceptible to warpage when heating unless the material is fairly thick. In most cases, for 3D printing, the warpage of most materials is negligible, and doesn't adversely affect the printed object. Other variations in the print will almost always be more noticeable than the relative flatness of the bottom layer.
In other words; don't sweat the technical specs for flatness. In case you're interested in learning more about Grade AA tolerance specs, check out this
PDF.
-David
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