The actual hole in the cowl is determined by the cowl light braces. After you mount the braces in the cowl the pilot hole is driled to the centered of that hole in the brace from the inside. Using that pilot hole, the hole for the lights is drilled from the outside. I don't have the size of the hole in the cowl right now. I will try and remember to look and post tonight.
once you get the hole marked and a small hole drilled to verify it.I wouldn't drill it to 3/4 ",I would use a Greenlee punch,they are available on line pretty cheap,it will make a perfect hole the first time