I enter weekly data into a spreadsheet with a summary page at the front.
After every week, when new data is entered, I want the formula at the front
to use the cell with the new data instead of me having to change the existing
formula.

For example, my data looks like this:

A B C
276
300
421
175
0
0
0

I need a formula that will automatically detect the last number 0 in column
C.

Thank you for your assistance!