Need some help here. I'm to the point with the Putz Walrus 18.5 where I have to determine the cockpit location. Putz says to figure the center of the cockpit at approximately 3/5 of the distance back. Other books, notably Ted Moore's Canoecraft, calculate seat placement based on paddler weight balanced against distance from center of gravity. Should I follow one of these two methods or is there another way to calculate? I'm concerned about this after putting the thwart in a bad place in my solo canoe. The likely occupants of the Walrus will be my wife and myself (neither dainty) and our 3 1/2 year-old son. Thanks, Tom.