How to Use Created Date “Count” in a Report Formula

1. Navigate to Reports tab, Click on it and Click New Report Button.
2. Choose Report Type and Click on Continue Button.
3. Add column to the report.
4. Under Group Rows, Select a field for your Parent Group and another field as your Subgroup. 
5. From the Columns pane in the overview panel, click , then select Add Summary Formula.
6. Fill the details:

•  Enter Column Name: [This will be the column header as you want it to appear on your report]
•  Formula Output Type Picklist : Pick the value you are going to have
•  Decimal Points: 0 [OR select the number of decimal places to be displayed
•  Display Tab: Select Specific Groups
•  Row Group: Amount [to calculate percentage according to rows]
•  General : Build your Formula – AMOUNT:SUM/PARENTGROUPVAL(AMOUNT:SUM, grouping_level
•  Click on the "Validate" button to check your formula is correct.

7. Click Apply.
8. Click on Save and Run.



See also: 
PARENTGROUPVAL and PREVGROUPVAL
Add a Summary Formula Column to a Report
Evaluate Groups and Totals with Summary Formulas