Ctrl+R fills right from the leftmost selected cell. Ctrl+D fills down from the topmost selected cell.
This way is a lot simpler and should cause fewer mistakes.
So, if you've got one column filled in for every row, and one row filled in for every column, you can quickly select the whole box and fill it down from the first row.
(Checking ... Libre Office 4.4 has Ctrl-D ... ah well.)