Perch_item_index in reverse?

I'm using perch_content_custom to display a list of events and numbering them with perch_item_index. I'm also sorting the list by date and in reverse chronological order using sort-order->DESC. When I do, the list is numbered 1-X thanks to perch_item_index. However, I'd like it instead to be numbered X-1. Is there a way to have perch_item_index number this list, but in reverse, with the most recent event having the highest number of total items, and the oldest event being numbered with 1?

Thanks!