I like to do as pappy said, just free hand the line following the grain on the center of stave. I've done a lot of carpentry, so I hold the pencil on center, and wrap a couple finders around the endge lightly. Then just slowly slide ur hand up and it will go with the bends and kinks. It works best on staves of more equal sized ends. But if it splits weird u can do it too, just a little more free handing.
If u have trouble just got ever say 6" and mark center of the stave. Then follow the grain and connect the dots