In the area that your wife lets you build bows in........string a tight line , e.g. a length of piano wire, or fishing line or single strand of B-50, TIGHT, low over the bench. Slide your stave under it. Arrange the stave so that the tight line is centered on the ends and in the middle. Or, you may have to adust it in all points to get the best out of the stave. Block up the stave so it NEARLY touches the line. With a sharpie mark the stave at the ends, middle, 1/3 points , where ever you wish. Now you have a "baseline" to work with. I concur with the expert advice above, this works if the grain of the wood follows the plank/board fairly well. In a raw stave split from a tree the rules are a bit different.
piper
piper