Let me add to Johns suggestions, as I go about the same as he. Not only use your eyes to follow the grain but also study the crown of the stave, most all have one. It as well the grain should be your centerline. I use a dashed line every 2-3" on the highspot and in line with the grain. Then come back with a circle making stencil and lay my bow out per that dashed center line.
I have had wayyyy too many staves not split the same way the grain runs full length and any tool that lays a line out according to that split profile wont be right every time.