Tablix help please!

I am currently using SSRS 2008R2. This is the script I am using:

SELECT

  a.name,

  DATENAME(M, a.Date)asMonth,

  DATENAME(YEAR,a.Date)asYear,

  DATEPART(YYYY, a.Date)as YearNum,

  DATEPART(M, a.Date)as MonthNum,

  COUNT(*) as [Total]

  FROM Database1.Table1as a

  WHERE a.name='active'

  GROUPBY a.name,DATENAME(month, a.Date),

  DATENAME(year, a.Date),DATEPART(YYYY, a.date),DATEPART(M, a.date)

  ORDERBYMonth desc;

I have created a Tablix that has the name of the product as the row group and the month as the column group. This works fine in that it gives a monthly count, but I would also like to see the monthly count expressed as a %.

My question is, how do I create another group in the Tablix so that the monthly % figure appears by each product?

From this, I would like to create a column chart that displays monthly % totals.

I am getting stuck because I have added a column to the end of my Tablix and have added this expression:

=sum(Fields!Total.Value) / Sum(Fields!Total.Value, "DataSet2") to calculate the %.

This just gives a year to date figure and appears at the end of the Tablix.

Any help much appreciated – thanks!