Would this not depend on the boat you are building and where the center of bouyancy of the boat is? Typically, your weight should be distributed right over the center line of bouyancy of the boat; an imaginary line down your spine when you are sitting in the boat should be the same line as the centre of bouyancy. When a boat is designed, a weight distribution is taken into account such as weight of typical gear front and aft and average weight of paddler. This might look something like 20 lbs for a day paddler and paddlers wieght such as 170 lbs for the average male. Were you not given any numbers when you purchased your plans?