SUMPRODUCT for YTD calculation HELP

I am struggling with a formula. I have tried sumproduct but can't get it to work but think its the correct approach.
I need a formula to calculate all charges dependant on a month entered. I have 2 sets of data
Actual and Forecast. An example of what I want to achieve is:

If Feb-12 is entered then the calculation will add up - Jan-12 and Feb-12 (Actual) then And Mar-Dec12 Forecast to give a YTD picture.

I have attached a very simple view of this but the data I am using is vast so I think SUMPODUCT is the way but as always your help and suggestions is greatly apreciated.

Many thanks