The fin marking guides from the Jim Z website fit!
The dowels are shown here after they got a shot of filler/primer and sanding with 400 grit.
There isn't much grain in a dowel, but they will be smoother if filler/primer is used. Look close, there is some gray primer left in the fine grain after sanding.
I prefer to glue it on with a small overhang, then sand it even with the fin edge.
The picture on the right shows how small and narrow the glue bead on the fin is. If the dowel ended up not on center, I can cut through If needed. The instructions suggest using the folded instructions under the fin as a spacer to center the dowel on the outside edge. I used my fingers to feel for the center position, running them down the dowel joint as shown on the left side photo.
On the right, the fit is better after some block sanding on the high spots.