What i would reccomend is looking at Bernoullis' equation. In this case it would be usefull to relate pressure and velocity. If you assume that the sides are sealed then you could use conservation of mass flow to deterimne the velocity at a particular point along the diffuser then determine the pressure.
what i can say is that solid fuel rocket motors typicaly have a diverging half angle ( angle between road and diffuser ) of 12 degrees.
also nozzles have what is called an area ratio or expansion ratio which is the ratio of areas between the throat and exit. This ratio is calculated by selecting some exit pressure - typically 1 atm. I imagine that you need a higher expansion ratio to create the desired low pressure under the car though so you might want to grab the ol thermos text book!
Im sure you knew all that stuff though.... so i probably didnt help much :s
Diffuser design on F1 cars is seriously compromised. The floor is stepped so the outer sections of floor are higher than the centre, plus the lowest point of the floor is raised by the plank.
Then the size of the diffusers tunnels are limited within the regulations, the outer channels start at the front of the rear wheels and must finish by the wheels centre line, and reach no higher than 125mm. The larger central tunnel can extend to the rear of the car, but a flat floor must precede it up to the rear wheel centre line. The space created inside this tunnel is limited by the gearbox and differential, plus to some extent the rear crash structure.
