As your data starts at row 5 (T5)then Zathras formula would need to be modified to adj the MOD-ROW reference as below
Also note the change to sum the 14 instead of 5th row
=SUMPRODUCT(T5:T6000,IF(MOD(ROW(T5:T6000)-4,14),0,1))
Remember Ctrl+Shift+Enter
Laurie