That's pretty much the way I do it. I first take a marker with a 1/4" spacer and move the 1/4" spacer along the deck while marking the cowl. I do use the sand paper in a similar fashion but I hold the cowl in position and move the sand paper. You might want to put tape or something on the deck to keep it from scratching. After the cowl is fit I then trim about 1/8" off of the front section of the cowl from the bulkhead to the bow to allow fresh air to enter the hull. I also cut out the cockpit area before I glue down both cowls to let the hot air escape. This the way I do it and is just a suggestion.