Hi,
FormCalc works well for this sort of calculation.
If all of the rows have the same name 'Row1', so that in the hierarchy you would see all of the instances of the row as: 'Row1[0]', 'Row1[1]', 'Row1[2]',... 'Row1[n]'. Lets say the objects in the column you want to add are called 'amount'. Then this FormCalc in the calculate event should work:
$ = Sum(Row1[*].amount[*])
This will also work if the row is repeatable.
If the rows are named differently, then you may need to manually add them together. Again this is FormCalc:
$ = Row1.amount + Row2.amount + Row3.amount
You can use JavaScript or FormCalc for this but if we stick with FormCalc:
$ = subTotal * 0.05 //in the calculate event of the feeAmount object (5 percent fee)
$ = subTotal * 0.21 //in the calculate event of the taxAmount object (21 percent tax)
$ = subTotal + feeAmount + taxAmount //in the calculate event of the total object
Hope that helps,
Niall