This may be simple for you guys but it's driving me crazy

I need to display, in a group, total percentage margin for a list of products.

It's accurate at detail level, but errors at group level.

Here's the list:

Desc:      Units:     Unit Cost:    Total Cost:  Total Selling: Margin: Margin %

Product 1   8.00       .413             3.30            6.72             3.42     50.83%

Product 2   6.00       .413             2.48            5.10             2.62     51.41%

Product 3 12.00       .413             4.96            7.32             2.36     32.30%  

Product 4   6.00       .413             2.48            4.92             2.44     49.63%

Fine so far, but when I group these items , summarize the totals and add formula for margin % as

sum({@frmMargin}) % sum({@frmSales})

this is what I get:

  Total Selling:   Margin:   Margin %
    
       24.06          10.84      50.83% 

It should be 45.11%

I've tried using the summarize button with ave. of margin %, but this is not accurate either.

What am I doing wrong?