For the benefit of all those who might follow, here is how I did it. By parts.

First, examine Vera Campbell's board

Vera.jpg
We need to zoom in, and so the inset does.

BeamDiagram.jpg
The beam we want to analyze is the blue rectangle. There is one fastener at each end. We must respect thin membrane limitations, or the deck will bulge between fasteners, green wavy line.

But plywood is not a monolithic piece of wood. It is comprised of layers. So a beam within the blue rectangle is a plywood layer! When computing beam deflections and the like, it is important to consider the beam height, as the stiffness goes by the beam height cubed. Forest Products Laboratory guidance shows this approach, as each plywood layer is analyzed separately, and the plywood thickness then in aggregate.

The force to apply to each layer is then my mass, subdivided by:

a) I have two human feet, divide my mass by two

b) Each of my human feet is 12 inches long. Distribute the mass found in (a) equally over contiguous beams, by the beam width.

c) Each layer of the plywood takes equal distribution of the mass in (b). Divide by number of layers.

Excellent. I now know the force to apply to each beam, and I know the beam dimensions.

Charts.jpg
The Elastic Limit chart shows how much each layer can stretch before rupture. The elastic limit of Okoume is applied to to the beam cross sectional area. Note that it will take ~800 lbf to rupture a layer of 6mm plywood, but 1200 lbf to rupture a layer of 9mm plywood. Why? Because both have 5 layers, and therefore the thickness of each layer is greater in the 9mm plywood.

Note in the chart Force per Layer, the same force is applied per layer, for the 6 and 9mm plywood because they each the same number of layers.

Okay, so use Fixed-Fixed beam deflection formulas to compute the deflection of a single layer of plywood, given the force located at the length-wise center of the beam. The third charts shows the result, the 6mm layer shows over 1" of deflection, while the 9mm layer shows less than 0.5" of deflection. The 6mm layer ruptures, yet the 9 mm layer does not! Why? Because the maximum deflection at rupture is ~0.725" for the longest (weakest) beam analyzed.

I could go thicker, of course, but then the weight penalty of a thicker deck comes into play. My volumetric computation places the board, in total, to be 61 pounds (Tom said ~70 to 75 pounds). The actual weight of the board, without the varnish, was 56.6 pounds. Adding varnish will make it very close indeed, which was 4.8 pounds in the two tins.

So my sense was that I would rupture or crack a 6mm deck (just under 1/4") but NOT rupture a 9mm deck (just under 3/8").

Your mileage may vary. There are other ways to analyze the decks, but this method lent itself well to the mathematical lofting I've created for the board.

Hopefully,that 10,000 foot overview gives a sense how I did it. I can provide the equations if interest exists.

Brad