I use an aluminum arrow shaft that has been sharpened and toothed on one end. I just stat at the outer most rib and drill all of them right away. this works well for making linkage ways in a wing for single servo aileron control applications as well. you can drill one hole then drill another by guiding the cutter to make the hole wide enough for the servo arm's travel by connecting the two holes with your exacto knife. it's always easier to get those holes made while the ribs are laying flat on the building table, but sometimes we forget !.