Is sigma_max the maximum of Max Principal Stress over the entire area or is it on an element-by-element basis? 

What about sigma?  There are three components: X, Y, and Z.  Do you need to integrate each one?

What element order will you be using: Linear or Quadratic?

What element shapes will you be using: Tri or Quad elements?

What level of precision do you need?  For example, a high precision approach will use the shape function of each element to integrate stress across the element while a simpler approach will use the element centroidal value.

For the simpler approach, try exporting the element centroidal stress into a spreadsheet then do the calculations in Excel.

Why do you want to do this calculation?  What is the benefit of computing this number?