The amount of closed cell foam needed will be different than the 14 cubic feet of open cell used from the factory.
You may need a fancy calculator.
(I think you need more closed cell foam for some reason...?)
there has been discussions about this in the past, and it would be easy, yet time consuming to figure out what it would take given the displacement weight (positive or negative buoyancy) of fiberglass, aluminum and personal belongings in the water.... it wont take as much in a water ballasted boat as it would in a weighted keel boat..
what is easy to figure is the displacement weight of foam.... per cubic foot, it is about 62lbs in fresh water and about 64lbs in salt water. a full 14 cf would be 868 lbs.... which may float a MAC 26...
and personally, if I were ever to get into an unfortunate situation where I got to experience if the boat floats right side up, upside down, or sideways, I think I wouldnt be so concerned with any misrepresentations or how much hype may have been in the sales brochure as i would with how much flotation was in the hull and being glad of having something to hang onto until help arrives, with a strong possibility of salvaging the boat.
some folks have said in the past that they wouldnt want to save the boat, to let the insurance have it... but most people love the boat they own and if the boat is in good shape, then why not save it.... especially a boat as basic as our macs.... they dry out perfectly, unlike some boats with all the fancys and wood through out which would take a lot more work to put back into shape.