Given the data, you'd set up the following three calculations:
Sales Reserve Amount: IF [Group] = "Sales Reserve" THEN [Amount]
Revenue Amount: IF [Group] = "Revenue" THEN [Amount]
% Sales Reserve/Revenue = SUM([Sales Reserve])/SUM([Revenue])
For this to work as described, neither the Account Code nor the Group can be in the overall level of detail of the view (Rows, Columns, Page Shelves or any shelf on the Marks Card, however the Filters Shelf is ok). If you need either of those two fields in the view, then there alternatives that can be created, the exact method would depend on your desired results.
Thanks for the tip!