Hi Keith, please can you save your data as a packaged workbook (.twbx extension) rather than a workbook (.twb extension). Otherwise the rest of us wouldn't be able to see your data.
The number at the bottom will probably be either total() or window_sum() of SUM([Market Share]^2). You should be able to wrap your on one of those as well
WINDOW_SUM did the job. To calculate this number at the supplier level, I had to add Supplier as a row and select Compute using -> Supplier (Sheet 1)
However, this of course gives the same answer for each supplier. To get one single number (i.e. the Hirschman Herfindahl index), I moved the Supplier dimension to the Pages shelf (Sheet 2).