ASME Code Case 2286-2 for External Pressure Design

[xema77]

I am trying to caculate the allowable external pressure on a cone section of a vessel using Code Case 2286-2. Section 4.1.1 provides methods for calculating De and Lce to substitute for Do and L in section 3.1 to determine Fha and Pa. My questions are:

1) What thickness shall be used to calculate Mx, Fhe and Pa? tc? tc*cos(alpha)?

2) Section 4.1.3 provides equation for Ch = 0.55*cos(alpha)*t/Do. Should this thickness (t) be tc?

Thanks in advance.