Using variables in VBA formula functions

Dear all,

As a beginner in VBA, I'm pleased to write my first post.
Here is my problem:
I want to use excell functions in VBA to retrieve data from a closed workbook.
My formula uses SumProduct function (because I understood that SumIf works only with an open workbook) and I want to create a loop to fulfill directly all the cells (from 0 to 16 of the column A) with the result of this same formula.
But when I use the variable "i" (indicating the row) into the SumProduct function using the quote (A"&i&"), I have a compile error "Syntax error".
Sorry it's probably basic, but as a beginner, I already spent hours on it without finding any solution...

This formula works without any problem when used in a cell of Excell, so I used perhaps the formula in the wrong way.

Please find my code below:

Dim Result As Range
         Dim i As Integer
         i = 0
         While i <= 16
             Set Result = Sheets("Report per cost center").Range("E" & i)
             With Result
                .Formula = "=SUMPRODUCT(--('D:\Personal Costs\2013\2. Database\[Personal Costs - 2013-V10.xlsx]Actual 2013'!$A$5:$A$52=A"&i&"),'D:\Personal Costs\2013\2. Database\[Personal Costs - 2013-V10.xlsx]Actual 2013'!$L$5:$L$52)"
                .Value = .Value
             End With
             i = i + 1
         Wend

I hope I am clear enough for your understanding.

Thanks a lot in advance for your support.

Regards,

Eric