Hi Norm.

Snap the chalkilnes.

The layout technique that Jack is describing is called square rule. What you are doing is laying out your joinery on an imaginary perfect timber inside the actual timber. Usually, you can treat one face of the actual timber as a face of the 'perfect timber' - the reference face. But the 'perfect timber', of course, has no crown. So you must create your own references using the chalkline. In my opinion, 3/16" on a 16' timber is enough to worry about. Good luck. CB.