Select rows of data to average base upon data being populated in adjacent cell

I am looking for a formula that I could put into a single cell below FY10, that would give me the average of the numbers for the same time frame as any numbers input into the adjacent column for FY11. The data for FY11 comes from a separate worksheet. This formula should also ignore zeros and the entire column includes 52 rows of data. So in this sample, the answer for the first two rows is 1218. However, once the 3rd row below FY11 is populated, then the average should look to the first 3 rows for FY10, and so forth for 52 rows.

Any assistance from anyone would be greatly appreciated, as I'm somewhat of a novice at Excel Programming. Thanks.