I have a relatively poor eye for assessing the flatness of a board, so I've started using a straightedge and feeler gauges when I'm uncertain. What's a reasonable standard for the flatness of a board? In other words, if I place a straightedge in a variety of positions on the face of a board and can't slip a .025" feeler gauge underneath it anywhere, would you consider that sufficient? .015"? I realize this likely differs depending on the intended use of the piece.