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Create a calculated field using a calculation similar to the following:
Then, place this on the label shelf. It may be necessary to change the way the calculation is computed. To do this, right click and select Compute Using-->desired output.
Hope this helps!
another option is to add a reference line, have it be a "Sum", and you can set the Line option to None, and format it so it is centered. see http://www.thedatastudio.co.uk/blog/text-label-beyond-default some examples