You want us to estimate your situation... When you look to Figure F-1— Appendix F Decision Tree , where is your position ?.
Acc to your querry, the answer is, calculate the weight of roof + shell and find the corresponding internal pressure.
It depends on what you mean, exactly.
The design pressure may be limited by the weight of the roof plates, by the top angle area, by uplift at the shell, by anchor bolt strength, by foundation weight or strength, by combinations of pressure and wind or pressure and seismic, by hoop stress in the shell, by membrane stresses in the roof plate, by reinforcing of shell or roof openings, by the arbitrary maximum pressure rating in the standard, etc. You'd have to go through the different options, determine which control the design, and work backwards to derive the maximum pressure.
