Auto-fill to last column of data and then to last row of data

Try on a copy of your data

Sub Sans()
    Dim Lrow As Long, Lcol As Long
    Lrow = Cells(Rows.Count, 12).End(xlUp).Row - 2
    Lcol = Cells(1, Columns.Count).End(xlToLeft).Column
    Range("L4").AutoFill Destination:=Range(Cells(4, 12), Cells(4, Lcol)), Type:=xlFillDefault
    Range(Cells(4, 12), Cells(4, Lcol)).AutoFill Destination:=Range(Cells(4, 12), Cells(Lrow, Lcol)), Type:=xlFillDefault
End Sub

See below

Sub Sans()
'dim statements to define variable type. save on memory usage
    Dim Lrow As Long, Lcol As Long
    'Last row statement. Rows.Count is the last row in the sheet.
    'End(xlUp)looks from the Rows.Count backwards until it finds the last row with data.
    '.Row turns the cell to a row number i.e row 43
    ' The minus 2 takes 2 from the row number i.e. row 41. THIS IS THE NUMBER YOU NEED TO CHANGE.
    'In your last example change it to 6
    Lrow = Cells(Rows.Count, 12).End(xlUp).Row - 2
    'Similar to the row explanation. The 1 is the row number.
    Lcol = Cells(1, Columns.Count).End(xlToLeft).Column
    ' ranges just the same as a normal range. The 4 is the starting row number, Column "L" is Column 12.
    Range("L4").AutoFill Destination:=Range(Cells(4, 12), Cells(4, Lcol)), Type:=xlFillDefault
    'As above
    Range(Cells(4, 12), Cells(4, Lcol)).AutoFill Destination:=Range(Cells(4, 12), Cells(Lrow, Lcol)), Type:=xlFillDefault
End Sub

Or I suppose another option is to change

Lrow = Cells(Rows.Count, 12).End(xlUp).Row - 2

to

Lrow = Columns(12).Find(what:="*", after:=Cells(4, 12), LookIn:=xlValues).Row - 1

But please note you will have to clear the contents each time before running.

Just to point out the clearing of data only applies if you are using the line below as it needs the empty spaces to work

Lrow = Columns(12).Find(what:="*", after:=Cells(4, 12), LookIn:=xlValues).Row - 1

If you are using something like the code below then it doesn't matter (unless you have less columns) as it doesn't need the empty spaces, because it looks for the last row with data and not the first row with data.

Lrow = Cells(Rows.Count, 12).End(xlUp).Row - 6