For a flat head, if the pressure is internal or external the stress in the head is basically the same, just the direction of bending is reversed. So, what PV Elite does is to take maximum of the internal and external pressure and use that for designing the flat head.

That equation is shown on the Internal pressure report. Recently we added a note in the external pressure report for flat head, pointing users back to the internal pressure report. There is no point in repeating the calculation.