Two Different Ways to Calculate Net Borrowing in FCFE Perpetuity

When finance textbooks address net borrowing calculation in free cash flow to equity perpetuity, they predominantly do it as the proportion of firm net investment financed by debt (Alternative 1). This method does not apply to all scenarios and requires a peculiar debt ratio. Alternatively, one can calculate net borrowing by applying sustainable growth rate to existing debt (Alternative 2). This method is broader and of simpler implementation. Alternative 1 can lead students, instructors and practitioners to confusion and, therefore, to misapplication and/or misuse. I suggest that neglected Alternative 2 becomes the standard method.