Having VBA Macros upload a formula to a Cell

I'm trying to write a sub routine that identifies a last name from a cell in column B, insert that name into the formula to identify which tab the formula is supposed to work with and continue to do that for each row in a loop. 

The routine I made makes the correct formula however, the formula only works as text.  The coding is posted below.  (I have used both Cells(I,3).Formula and Cells(I,3).FormulaArray but neither seems to work, please help?

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    Dim New_set As String
    New_set = Range("A12")
    Dim Hours
    Dim Men As String
    Hours = "=IF(Last!$B$6>0,(Last!$B$6-Last!$B$5)-SUM(Last!$B$7:$B$10),(IF(Last!$B$5="","",$C$1-Last!$B$5-SUM(Last!$B$7:$B$10))))"
    If New_set = "Load Formulas" Then
        For i = 13 To 17
            Dim LastN
            Dim p
                LastN = Cells(i, 2)
                p = Mid(LastN, 1, InStr(1, LastN, ",") - 1)
                Cells(i, 3).FormulaArray = Replace(Hours, "Last", p)
        Next i
    End If
    Range("A12") = " "
End Sub