Calculate YTD returns, varying start of the year using dropdown

Hi

I would be grateful for someone's help, this problem is driving me crazy...

I have a portfolio with month end performance figures - column A shows the month end dates, column B is the value of the portfolio as at month end and column C is the change in return each month. Cell F1 is a drop down list of all the monthly data points, which is a dynamic range of column A

See attached spreadsheet.

I have a series of calculations on the same worksheet which shows the 1 month, 3 month, 6 month, year to date and returns since inception which work off the date in the dropdown list. Therefore as I change the date in the dropdown, the output changes for the 1 month, 3 month etc.

The problem I have is with the year to date calculation. The formula is below. At the moment, the begining of the year is fixed to 31/1/12. I need to be able to change the begining month for the ytd calculation if for example I change the end month to 31/5/11. The current formula for ytd is

=SUM(INDEX($B:$B,MATCH($F$1,$A:$A))/$B$112)-1

As you can see, the begining month is hardcoded to B112 which is 31/1/12

Therefore if for example I change the date to 31/8/11, I need to be able to see the year to date return for 2011 and not reference 2012.