How can I make calculations based on smart category summaries in Numbers?
While it's not (currently) possible to place custom values in the column you desire, it is possible to refer to the operands elsewhere, such as by using a secondary table to refer to the cells in question. To make a formula point to a certain category’s subtotal, hold ⌥ option and click on the desired cell.

Unfortunately, this method does require the categories to be entered manually, so it may not be suitable for all situations.

Here is a demonstration using your example table, with a secondary table holding ratio values for each of the categories: