M_Boats: m-15 floatation

While putting my m-15 away for the winter, I finally opened up the bow compartment and found that there was no flotation foam under the cover.  I had already noticed that there was no foam at the stern.  I am planning to put in two part closed cell foam in the spring, and wonder if anyone has an opinion as to how much foam I will need, or how much weight I need to calculate to determine how much foam.  Research seems to show that you don't need pound for pound for fiberglass, but you do for metal such as ballast, motors and spars.  So how many cubic feet of foam floats an m-15?