VBA Calculate average of stocks

Dear reader,

I need to calculate the average return of 600 stocks over a period of 3 months, i need to repeat this numerous times and pick the stocks that have the best 10% return. So for every three months pick the best 10% of stocks, and repeat this 20 times which creates 20 portfolios. I was looking for a VBA code that can help me do this.

As a test I created a new excel file with a small amount of numbers, and the only thing I am able to do is calculate the average of 1 stocks for 2 months...

Sub average()
For i = 1 To 1
av = (Range("a1") + Range("a2")) / 2
Range("a6:h8").Offset(i) = av
Next
End Sub

I find it difficult to use VBA where it gives the solution on the excel sheet, and how to make this a code which you can use multiple times (moving average). I can only use the cells I named and I can't let it roll down..
I am not sure how to use a loop here...
Please let me know if anyone understands this or if you need more information.

Can you attach a copy of your spreadsheet explaining what it looks like at the beginning and add a sheet which shows the result? To attach a file, click on Go Advance scroll to the Manage Attachments button, browse to the required file, and then push the Upload button.