Code:
{series}/{series_index:0>2s}-{title}_{authors}
Prepend the Index to the title. that make it sort in series # order
having Author before, just messes things up if having different authors (or combinations) writing in the series.
Sort goes from Left to Right so the significant part is: {series}
/{series_index:0>2s} (I put all of the series in a separate folder), but you could replace the green with any allowed character