I. Have not read the whole thread as I am on a very small computer at the moment, but is it proper to consider that the pressure at 2 3/4 is less than at 2 1/2 in a normal shell ? If so the calculation of the safe stress at each distance should be made, and that seems to say the wall thickness at the 2 3/4 distance could be thinner than at the 2 1/2 and be equally as safe in each area.

http://www.bakercollectors.com/