Calculate running total in one of the Grid controls

Hello,

I need to calculate a running total for a column. I have done this in the past (with a different control) using an unbound column (Z) with a formula that adds column (X) in the same row (the next value to be added to the running total) to the current running total (Z) held in the previous row.

So 3 questions:
1. Which Grid do I use, DataGrid or FlexGrid
2. How do I create a formula that references another column in the same row and itself in the previous row: Using arrays, for row 100: RunningTotal[100] = SomeValue[100] + RunningTotal[99]
3. Which event do I code the formula (if formulas are supported) otherwise, which event do I set the RunningTotal.

Thanks in advance for any guidance!