Flush-Type Connection(API 650): Meridional Stress is 1/10 Times Design Stress, Sd

Dear all,
I am working on Tanks having flush type nozzle connection. I have douts the way i am interpreting the requirement as per clause API650 5.7.81b, which says: "The vertical or meridional membrane stress in the cylindrical shell at the top of the opening for the flush-type connection shall not exceed one-tenth of the circumferential design stress in the lowest shell course containing the opening"My solution:
API620 gives us the formula for meridional force/unit lenght,T1(5.10.2.5). Now using T1 and actual thickness of first shell course, i calculated the meridional stress using thickness formula of 5.10.3.2, and compared this vertical stress with my design stress, Sd.

In my case the shell thickness is 6mm and and using the above stated method my required thickness comes around 32mm. So this means i have to use this much thickness of insert plate, but this thickness is too high.

i also tried basic stress formula : pr/2t and also used ANSYS. With ANSYS even 16mm thickness is ok

Please I need urgent help that what is the right formula or method for this clause


Thanks
Inam