So how flat is flat for a building board? I have a table that has a very slight bow in it that I cannot seem to remove via shimming. At this point I have tried putting a 36" straightedge along various axis of the table and find one axis where there must still be a dip - it is not large enough to put a peice of paper through, but I can tell it is not quite flat because if I wiggle the ruler (a flexible metal yard stick) there is more resistance at the ends of the ruler than there is in the middle. Also looking at the shadow the edge of the ruler creates it looks (perhaps an optical illusion?) that the shadow is just slightly bigger in the center.

With all the suggestions for building boards I see on here, from glass to hollow core doors to laminated shelving, in order to acccurately build a plane how flat does it have to be? I do understand that the flatter the better, but is there some degree of precision I need to achieve and some point of diminishing returns where getting any flatter doesn't help?

This is my first built up plane and is a 115" wingspan sailplane - I would assume any errors along the way will translate to big errors along 9 1/2 feet of wingspan!