: This question may be silly, but for a center line that runs through
: the blade tips, how do you draw an 84" straight line
: through an entire paddle blank?

The easiest way is to start by drawing the lines on the blank before you do any cutting or tapering. You can use a thread running from one end to the other and make marks that you connect with whatever straightedge you have (I use a 36" steel rule). Make sure you draw the lines across the ends too, as once you start cutting and tapering, you'll remove the marks on the faces and edges, but you can use the end marks to reestablish your centerlines.