In the standard report “Campaign Analysis” there is a column for % Return. The formulas are not visible in the resulting spreadsheet, just the result, but it appears that the formula being used is # Transactions divided by # of Packs Sent. It would be more accurate to have the formula as  # Donors divided by # of Packs Sent, as transactions increase more over time than donors, through regular and recurring givers. As the formula is preset into the report there is no way to make any change to it.