Not sure it makes any difference to the discussion of what loads to shoot but, hoop stresses are in fact highest on the inside face. You cant figure it out from the Pr/t equation (a simplification) You need a different method.

Hoop stress = ri^2 x P/(ro^2-ri^2)x(1 +(Ro^2/r^2))

ri = inside radius, ro = outside radius, r=spot in question (r=ri if you want the max stress)