if you know the row and
a) there are no blanks in the data you can use
range("A1").end(xltoright).column
b) there are blanks in the data you can use
range("IV1").end(xltoleft).column
in your example both of these will return 20 (the number equivalent of T)
so you can do something like
cells(1,(range("IV1").end(xltoleft).column + 1)) = "last"