As a mat is inflated the deck and hull pieces become separated, as they separate the sides come in, quite dramatically in fact, some Mats can shrink by 5 inches.
I want my mats to be the shape I want them to be, its all about getting it right, the right shape, the right volume, rather than leaving design to random chance.
So starting with the right shape, factor in the number of pontoons, the curvature of the pontoons, the height of the I beams and the stretch of the fabric you get the original shape which is the intended shape of the inflated mat and the new larger outline for the uninflated mat.

I'm going to do another completely clear vinyl mat so the valve is glued into a clear piece that acts as reinforcement.