The reason that the grand totals are not returning as expected is because totals use whatever aggregation is on the measure. It is not necessarily a sum of whatever is in the row or column. Therefore, since the sum is being applied first to Amount, the Amount is being summed and then the absolute value is added. By adding the window_sum or running_sum the last value in the row/column will be correct, however, the grand total will revert to the aggregate on the measure.
Currently, the best solution is to get the desired grand total on one worksheet and the desired details on a separate worksheet and put the two on a dashboard together.
Hope this helps!
That's an awesome solution! I've never thought of that...thanks Joe!