This also takes care of when No series
Code:
program:test(field('series'),strcat(
substr(field('series'), 0, 1),template("-series/{series:subitems(0,1)||/}{series:subitems(1,2)||/}{series_index:0>2s}-{title}_{authors}")
),template("00noseries/{author_sort[0]}_authors/{author_sort}/{title}-{authors}")
)
(Original Credit to chaley)