to calculate if we need intermediate wind girder or not according to API650

5.9.7 Intermediate Wind Girders
5.9.7.1 to calculate the maximum height of the unstiffened shell

t = as ordered thickness, unless otherwise specified, of the top shell course (mm)

5.9.7.2 to calculate the height of the transformed shell

t= as ordered thickness

I think that we have to calculate according to corroded thickness as it is the worst case.

Please advise.