> or = to 0 includes blank cells

I'm making a scorekeeper for a game of 501 darts. It's basically two columns; one saying Remaining Score and the other saying Scored.

In order to prevent all the cells in the column from showing the Remaining Score when it's only required in the most recent cell, I came up with this formula:

=IF(Y6>0,X5-Y6,"")

Which says... If the score is greater than zero, then subtract scored from remaining score, if not then the remaining score cell will be blank.

The problem with this is that it's actually quite common to score zero in darts when shooting for doubles to end the game, but when I made the formula say greater than or equal to zero, it also counted the score cells that were blank... Meaning the remaining score was in all the cells anyway.

How do I get around this?