[RINA] Bottom Pressure for planing boat

I am using RINA(Registro Italiano Navale) scantling rule to calculate bottom pressure and etc for FRP planing boat.
The rule says,

P2 = 15*(1+av)*(W/(L*Cs))*g*FL*F1*Fa

But I don't understand what Cs means.
RINA Yacht 2007 book says, Cs is "Support contour of the yacht, in m, defined as the transverse distance, measured along the hull, from the chines to 0.5L. For twin full yachts, Cs is twice the distance measured along the single hull."
Please help me, this Cs changes the pressure a lot so I have to define the value correctly.
Thanks.