If you structure the data in reverse order and add move your total function to the top of the table directly under the headers then you only need to sum the cells when you hit 30. You can use an INDEX, MATCH combination to do this. It saves using the SUMPRODUCT formula. I recommend using the MATCH at the top of the flag column as this will be a calculated constant that you don't need recalculating in each cell. In a large workbook this will save you calculation speed.

Peco was right in his comment, in isolation. In large workbooks with abundant data the length of the array in repeated array formulas will affect your calculation speed and usability of the file. How much depends on repetition and volume of data. INDEX uses an array so take care if your workbook is a large one.