Determining the Used Range

Sorry. The LastNonBlankCellAddress UDF in my comment below should be named LastNonBlankRowColAddress because the cell might actually be blank.

The following UDF will return the absolute address of the last non-blank cell in a range of cells (Target):

Public Function LastNonBlankCellAddress(Target As Range)
    Dim R As Range, LastRow As Long, LastCol As Long
    Application.Volatile
    Set R = LastNonBlankCellIn(Target, xlFormulas, xlByRows)
    If R Is Nothing Then _
        LastNonBlankCellAddress = CVErr(xlErrNA): Exit Function
    LastRow = R.Row
    LastCol = LastNonBlankCellIn(Target, xlFormulas, xlByColumns).Column
    'dynamic array formula spill can result in cells
    'that have a value without a formula or constant
    Set R = LastNonBlankCellIn(Target, xlValues, xlByRows)
    If Not (R Is Nothing) Then
        LastRow = IIf(R.Row > LastRow, R.Row, LastRow)
        Set R = LastNonBlankCellIn(Target, xlValues, xlByColumns)
        LastCol = IIf(R.Column > LastCol, R.Column, LastCol)
    End If
    LastNonBlankCellAddress = Target.Parent.Cells(LastRow, LastCol).Address
End Function

Private Function LastNonBlankCellIn(Target As Range, _
    LookIn As XlFindLookIn, SearchOrder As XlSearchOrder) As Range
    Set LastNonBlankCellIn = Target.Find(What:="*", After:=Target(1), _
        LookAt:=xlPart, LookIn:=LookIn, MatchCase:=False, _
        SearchOrder:=SearchOrder, SearchDirection:=xlPrevious)
End Function

John could use the UDF in a cell formula like this:
    = "$A$8:" & LastNonBlankCellAddress(A:N)
If a relative address is preferred, use this:
    = "A8:" & SUBSTITUTE(LastNonBlankCellAddress(A:N), "$", "")
LastNonBlankCellAddress accounts for cells that are hidden or have a formula returning null text (""). The Tip's last formula ignores cells with null text.

The first two formulas work in specific circumstances, but with caveats as described. They work best if at least one column has no blank cells and the count is based on that column. The last formula given works without those caveats, but I think the worksheet can be designed in a way that may be easier to manage later, even though it is a bit more work to set it up.

My suggested approach is to first create a table for the area where the data will be entered. Such table can be created with some data already entered, including heading row with labels. After creating the table, if you wish you can turn off the auto-filter (on Data tab of the Ribbon there is an icon for that), as well as change/remove any shading and borders that are automatically imposed by Excel. The benefit of such approach is that if you enter new data in the row directly below the last row of the table, the table will expand automatically. If you enter the additional data further down the spreadsheet leaving some rows empty, the table will not expand automatically, but you can easily adjust the range of the table by dragging the small triangle at the bottom right of the table. This way you can also exclude some data from the table. And of course you can enter/update the data within the table. You can also insert or delete rows from the table except the header row, and the table range will adjust automatically.

Once you have created the table, Excel gives it a name, typically "TableN" where N is a counter of the tables created. You can find that name by clicking inside the table and opening the Table Design tab of the ribbon: the information is the the Properties group at the very left of the ribbon. Note that the table name refers to the range of the data in the table, excluding the header row.

Now, to use all that in the formula that John wanted, you can type in = "A8:N" & (ROWS(Table1) + 7). (adjust the table counter if necessary).

The Tip's last sentence says, "...if you are using an older version, you'll want to enter it as an array formula (Ctrl+Shift+Enter)." I don't think this is necessary because the formula does not return an array.