Hi
There is no variable for last matched back, so probably best to use a lay trigger, with last_traded in the price field and the appropriate distribute loss formula in the amount field. Your conditions should include: selection's traded_last is greater than 100. However, I think the volume traded at a specific price is only available if the ladder interface is open on that selection.
Quote from the manual: "The amount of money matched in the market on this selection at the price specified by {price}. For example, traded_4.5 will return the amount of money traded at the price of 4.5. This variable will only work for selections monitored in the Ladder interface, because obtaining the information about traded volumes requires a separate call to BetFair which is only made if the Ladder is open”