Calculating Percentages

I need help with a formula. I’ve simplified my problem in the following example:

Cells A1:A28 contain each day of February.
Column B shows my chess game results on the days in column A. I didn’t play every day, so there are blank cells in column B.
“1” represents a win and “-1” is a loss.

SEE ATTACHMENTS - xls OR jpg

Starting in cell C15, I want to know my winning percentage for the last 10 days IN WHICH I PLAYED –not merely the last ten days. So formula in C15 must go back to the first row to inform me that my winning percentage is (including 15-Feb) =60%. I’d like to drag it down to keep an ongoing record which won’t change until I played again on 18-Feb =70%.

I APPRECIATE ANY HELP!