在下面的示例中，如何在单个用户总计中添加公式，因为用户的行数可能会有所不同。
+------+--------------+---------+--------------+-------+
| Name | Date | Billable| Non-Billable | Total |
+------+--------------+---------+--------------+-------+
| abc | 06/23/2012 | 860 | 10 | 870 |
| | User Totals: | 860 | 10 | 870 |
| xyz | 07/12/2012 | 45 | 0 | 45 |
| | User Totals: | 45 | 0 | 45 |
| ccc | 09/19/2013 | 165 | 35 | 200 |
| | 10/15/2013 | 240 | 0 | 240 |
| | User Totals: | 405 | 35 | 440 |
| | Grand Totals | 1310| 45 | 1355 |
+------+--------------+---------+--------------+-------+