Ron I didn't predrill them ,I already had built the wings when I decided to add the wing tube.Once I figured out were the tubes had to be with the dihedral I cut the holes using a hole cutter.I cut the holes oversized which in turn gives you some play to make final adjustments.I think I used a 1 1/2 hole cutter(with an extension) and then used a slightly larger drum sander.(I just added sticky back 80 grit? paper that Tower sells and wrapped it around the hole cutter).Once you have your tubes inserted you assemble the whole plane using clamps to line everything up.On a level surface you level the plane,then you prop up your wing tips to the proper dihedral.Once everything is lined up you can epoxy(45 min or longer type) the tubes in place using the 1/2" balsa blocks on each side of each rib.
Figuring out where the holes go involved;mounting the wing on one side only,set the dihedral,and using the carbon fibre inner tube ,inserted through the fuse,through to the wing to mark the location for the next rib to be drilled.Once you have 2-3 holes drilled you know where the rest will go and you can pull the wing off and continue drilling holes.Its time consuming but it works.You can see in the pics were I added additional wire braceing across the top in the cabin instead of those dowels.Very stong setup,you can do loops and spins all day with this ..hope that helps Ron