The problem IMHO is being addressed the wrong way here.  Note:

1.  The PIC for the last field has to accommodate the largest
    possible length.
2.  One needs a record code somewhere in the record indicating
    the format of the record giving a clue to the length of the
last
    field.
3.  Failing that you can pre fill the last field with a character
    that will never appear in the input record and then after
reading
    the record search the field from left to right looking for
the
    fill character.  Ergo you know the record length and what
part of
    the field contains data from the file.

That's how I used to do it.  Maybe others have a better way?

Nick D.