Flat is relative for a table like this, Dave. As long as it "looks" flat and is well supported, something of that stature doesn't need to be ready to be a machine shop reference surface. Leveled so it looks like a continuous surface is more important and that's one reason I suggested the wide belt treatment once you have it glued up...getting the whole schmegeggi the same thickness so it looks like it's a continuous surface like I just mentioned.
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The most expensive tool is the one you buy "cheaply" and often...