Apply Parts discount on a case over all Parts Sales Accounts.

For a case where the price is fixed on a Service Group and it includes (for example) Parts Sales and Oil Sales, when the Parts Adjustment is made to reduce the Sale, it only adjusts the Sale of Parts. Often the Oil Sale is much larger than the Parts Sale. I would like the Adjustment amount to be split over the two Sales Accounts in ratio of the Sale Amounts to each account. This will help keep the Sale of Oil better reflect the actual sale/profit.