I have a summary worksheet that totals up some values on one of my other
worksheets. The problem is that I have to keep changing the range of my
formula because the data on my other worksheet changes daily and more rows
are always added. How can I set up my formula to automatically go to the last
row it finds data in on my other worksheet?


Here's an example of my formula:

=SUMPRODUCT(--(MONTH('PC-01'!$O$2:$O$20000)=MONTH($A11)),--(YEAR('PC-01'!$O$2:$O$20000)=YEAR($A11)),'PC-01'!$AB$2:$AB$20000)

As you can see it's set up from O2 to O20000. I keep having to change the
O20000 since new data is always added. Is there an easy way to automate this?