FEA outputs results for the elements (and nodes). if you want to average over a group of elements, since you don't seem to be using an interface (but are reading the output file directly) i think you'll have to do that by hand.
i'll assume your group od elements represents a cross-section of the model. so wouldn't the load be the applied load (and the average stress = P/A) ? or are you just checking to see if the FEA is producing correct results?
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