Formula for period-to-period percentage change. Use a date grouping, such as created date (group by week, month, etc. Then use a report summary formula such as: (RowCount – PREVGROUPVAL(RowCount, CREATED_DATE))/PREVGROUPVAL(RowCount, CREATED_DATE) Date group must be sorted ascending.
Percentage of grand total summary. Found this useful when showing group record counts (Matrix reports) as percentage of the total: