craigd
I use a Starrett ruler to check flatness: it is thick enough that it balances on edge, shine a light behind it and you can notice well under a thou. That said, I don't like perfectly flat soles, they are too sticky (or "wring"). A "flaked" (a side effect of scraping) sole is probably close to perfect; overall very flat but has wide spread "imperfections" (maybe like unfinished wood is never perfectly flat because of the grain). I never go much under 100 grit and don't bother to take all the mill marks off the soles of my planes.