After completing your work say in range A5

50 - Highlight it or Select it.
Then at the menu, Edit, Goto... Special button.. Click Blanks (on
left-hand side, then OK
The first available blank cell will be active, enter a 0 (zero) in it BUT
before pressing the enter key hold down the Control key AND the Enter key.
This will fill all selected blanks with the 0. While previously blank cells
are still highlighted/selected go to format cells your choice and complete
(numbers, ))..
HTH