Enter the length of the outer cylinder to the nearest body flange or head, or to the centerline of the convolute.
TEMA Paragraph RCB 8-21 includes the following note: lo and li are the lengths of the cylinders welded to the flexible shell elements except, where two flexible shell elements are joined with a cylinder between them, lo or li as applicable shall be taken as half the cylinder length. If no cylinder is used, lo and li shall be taken as zero.
Entering a very long length for this value will not disturb the results, since the TEMA procedure automatically takes into account the decay length for shell stresses and uses this length if less than the cylinder length.
This value is shown in the figure below as 'lo'.