"=AVERAGE(" & .Range(.Cells(2,5),.Cells(rw,col).Address(0,0) & ")"

That's the ticket. 280 characters, but it works.

xlapp.Workbooks(strXlsFile).Worksheets(sn(i)).Rang e("C2:C" & lr).Formula =
"=STDEV(E2:" &
xlapp.Workbooks(strXlsFile).Worksheets(sn(i)).Rang e(xlapp.Workbooks(strXlsFi
le).Worksheets(sn(i)).Cells(2, 5),
xlapp.Workbooks(strXlsFile).Worksheets(sn(i)).Cell s(2, lc)).Address(0, 0) &
")"

Thanks for the help!