Ratio of Group Level Data against Summary Data - Crystal Reports

Question

I having trouble finding a way to ratio Summary data against Group Level data (not the detail Level)

A simplified version of the problem is as follows:

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Say, I have two tables with a one to many relationship:

stock LEFT OUTER JOIN sales ON stock.barcode=sales.barcode

The report is layout is as follows:

GroupHead2 |
GroupHead1 | Sales$                       %             Stock$      %
Details    | Sales
GroupFoot1 | a=Sum(Sales,{Group1})       a/b            c=Stock      c/d
GroupFoot2 | b=Sum(Sales)                               d=Sum(Stock)

I can get the ratio a/b to work as follows:

a/b = Sum({Sales},{Group1})/Sum({Sales})

But "d" is a running total (made with the running total wizard) because I only want the stock on Hand for each barcode Vs the sum of sales for each barcode.

Thus if I try to get the ratio "c/d" I receive an error.

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EDIT: Output looks as follows - Say:

Sales$   %            Stock$   %     
200      13.3         50       100
300      20           20       28.5 
800      53.3         30       30
200      13.3         10       9.1
----------------------------------------------------------------
1500                  110

Note the Percentage stock is wacky because it is recalculating the total (running) on each line instead of using the total value (eg 110)

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EDIT 2:

I saw this as a Similar Question and was hoping someone could offer the adaption (which has escaped me) of this answer to suit the above scenario.

The answer is as follows:

Sum ({ numeric field }, { grouping item }) % Sum ({ numeric field })

The problem is slightly different to mine as the { numeric field } is introduced at the group 1 level thus it only needs to be evaluated once per group 1.

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I'm guessing there maybe a way to get "d" without using a running total (?) or a better way to make the ratio "c/d"?