Referring to a subtotal in a formula

I have set subtotals into a large table of data, and wish to use a formula that refers to the subtotal of a column. The formula is a simple one that creates a percentage value of the figures higher in the same column as the subtotal value. The issue is that the table will gain data and have data moved around, thus I need a formula that will always refer to the subtotaled cells no matter where they get placed or what row they end up on with data manipulation. This is for a report that will allow my company to calculate how we bill a client, so it must be a reliable one, even at the cost of being extremely complex.

Josh