SUM when using a looping macro of variable length

Hi guys,
I'm making a spreadsheet to record call stats in a call center.

The call center has fluctuating employee levels due to the norm (Promotion, employees leaving etc.)

I've therefore made a looping Macro to fill out the call stats where the user enters the number of employees they are filling the report out for.
This number is "x" and is the basis of the looping macro (For I is 1 to X...)

However, i need to do some sums on a column of data of indeterminate length.

I have come up with

ActiveCell.FormulaR1C1 = "=SUM(R[-X]C:R[-1]C)"

The second digit will always be -1 as this is going to the bottom of the list. However, is there a way to get excel to take the first value as -x?

I've always used R1C1 format as this tends to be the format that my Excel automatically registers when I record some of the more mundane parts of the macro.

If it wasn't then it would be easy, as this value would always be 4 (A4,B4....)

Thanks Alpha,
I was missing " & X & "