How to specify file name in a formula used in VBA when looping through multiple files?

I'm currently looping through multiple files using the following code:

    Dim wb As Workbook, sFile As String, sPath As String
    Dim itm As Variant
    Dim strFileNames  As String
     
    sPath = "C:\.......\Annual and Quarterly Budget Data\"
     
    ''Retrieve the current files in directory
    sFile = Dir(sPath)
    Do While sFile <> ""
        strFileNames = strFileNames & "," & sFile
        sFile = Dir()
    Loop
     
    ''Open each file found
    For Each itm In Split(strFileNames, ",")
        If itm <> "" Then
            Set wb = Workbooks.Open(sPath & itm)
            '' DO LOTS OF CALCULATIONS HERE
        End If
    Next itm

Inside the loop, I have the following code, which does a SUMIF formula in one of my columns:

BD.Sheets("Sheet1").Cells(Rows.Count, 6).End(xlUp).Offset(1, 0).Formula = "=SUMIF( 'P:\Actuary\Cash Flow Forecast\Annual and Quarterly Budget Data\[ECMQA 2012Q1.xls]Sheet1'!$D$13:$D$" & LastRow & ",D" & BD.Sheets("Sheet1").Cells(Rows.Count, 6).End(xlUp).Offset(1, 0).Row & ",OFFSET('P:\Actuary\Cash Flow Forecast\Annual and Quarterly Budget Data\[ECMQA 2012Q1.xls]Sheet1'!$D$13:$D$" & LastRow & ",0,MATCH(E" & BD.Sheets("Sheet1").Cells(Rows.Count, 6).End(xlUp).Offset(1, 0).Row & ",'P:\Actuary\Cash Flow Forecast\Annual and Quarterly Budget Data\[ECMQA 2012Q1.xls]Sheet1'!$D$12:$R$12,0)-1))"

Since I am looping through all of the files, how would I change the part of the code with the file name (code below) to pick each new file and not a fixed one like I have above?
LastRow is 234 so the file and cell reference is:

'P:\Actuary\........\[ECMQA 2012Q1.xls]Sheet1'$D$13:$D$234

Each file has the same exact format.


PLEASE HELP!
Thank you!

Thanks I'll test it out when I get back to work on Monday!