You can use the "Status" tab of the dataset to add metrics that will compute after each build of the dataset. If you click on Status > Edit you can add metrics. For columns, you can select "Sum" as one of the metrics that is calculated:
If you return to the "Metrics" tab, you can then add this metric (and any other metric you are interested in calculating) and it will display on this screen:
You can also calculate SQL Probe metrics if you ever need to create a more complicated metric.
Let me know if you have any questions about this approach!