Thanks for looking at this. I have checked through the help (F1) but somehow
or rather, just can't find the solution:

What I wanna do is to substitute the 5450 with 6001 (the formula below). Coz
the range has increased from 5450 to 6001. If I can slot this in, then I
won't have to set the range everytime it grows.
* CountA(Dbase!A:A) equals to 6001


Substiture K5450 and F5450
=SUMPRODUCT(--(Dbase!$K$1:$K$5450=VLOOKUP(DATE(YEAR($B$25),MONTH ($B$25)-1,DAY(1)),Calendar,2,FALSE)),--(Dbase!$H$1:$H$5450=$B19),Dbase!$F$1:$F$5450)

Thanks.