It will be a while before I get to the cowl on the FM. However, I will describe the technique that I use. Mount the motor. Make four or five 1/8" wide strips of 1/16" balsa about 1/2" long. CA these evenly around the circumference on the aft side of the spinner back plate. Now center the cowl nose ring on these little strips and CA in place. Mount the spinner back plate and nose ring to the motor. Build your cowl as necessary mate with the nose ring. Once completed and sanded, your cowl and spinner will be perfectly spaced and centered.