Depending on when you got your kit, this may all be a moot point as Zenith now supplies the entire vertical fin already match hole drilled.
Drilled the spar on both sides to #40...no big deal. Then opened skin and clecoed in the big rib to the spar. Drew a line 90 degrees to the spar on both edges of the rib. Used a 12" #40 drill bit to drill through the pilot holes and into the skin. Pick a hole in the middle first and then cleco it. Cleco as you go. You need to bend the leading edge of the skin up to the curve of the rib as you move along. To get the other side done, cleco the other edge of the skin to the spar and then flip the fin so the side of the rib you just drilled is up. You'll need to support the fin underneath to make sure the clecos are off the table. Now move your 12" drill bit through the holes in the rib you just drilled, look inside the fin and guide the drill bit through the hole below., meanwhile pressing down on the skin to ensure accurate transfer of holes from rib to skin. Cleco as you go.
