Top Wind Girders per API 650

[TankDude]

I need some qualified opinions on the following: there is a note at the bottom right of page 3-39 in API 650 that states (in relation to top wind girders): "For tank diameters over 200 ft, the section modulus required by the equation (from 3.9.6.1) may be reduced by agreement between the purchaser and the manufacturer but may not be less than that required for a tank diameter of 200 ft." The way that I interpret this is that regardless of the tank diameter beyond 200 ft, it is acceptable to size the top wind girder as if it were a 200 ft dia. tank (pending agreement). Does this seem correct? Does anyone know the thinking behind this? Could it be due to the relatively flat wind profile for large diameters, which changes only slightly beyond 200 ft? Is there residual strength in the shell plate unaccounted for that allows this? I am curious to read the responses to this one. Thanks.