I found I could very accurately position a #40 (135deg split-point) cobalt drill bit from Aircraft Spruce. I didn't experience any tendency to wander and they are ready for a cleco without updrilling. Like Gill said though, you can easily get the feel and stop drilling when the bit makes it through the skin, then you'll note a tiny dot with on the sharpie line or just either side of it and you can slightly tweak the substructure
I found the challenge was'nt in the drilling but laying out your rivet lines/spacing on the skin to fall in just the right places on the substructure. It's a bit unnerving at first but just measure and confirm two dozen times like I did before you put the drill to the skin. Watch the location of the notches in the ribs, spacing must be adjusted to maintain edge distance.