You are bound to have more than one go crooked if you do more than is needed. To get a good straight hole I suggest making yourself a drill bar by taking a piece of 1/2-3/4 inch metal, size it to lay flat in the surrounding area of the hole, use a drill press to drill a hole in the bar that is 1/64th larger that the drill size you will need for the helicoil. Then drill your repair hole using that bar to keep your drill straight. Verify the straightness of the hole in the bar with a square first.