Co can be estimated from (dP/dx)/Velocity^(C1), This relationship is best determined from data that provides the pressure drop across your porous medium as a function of different velocities, as a single point is insufficient to determine the trend in dP with velocity. Plot dP/thickness vs. velocity to get an idea of the shape of the curve. The curve shape can be used to determine the velocity exponent, C1. Then, fit Co according to the magnitude of dP/dx.