Personally, I don't use Calibre for my manga. I just keep them in series folders as ZIP/CBZ and KOReader seems to just use the filename as title.
This works in template tester. Dunno if it'll work in the plugboard.
Code:
{tags:'contains($, 'Manga', '', strcat(field('series'), finish_formatting(field("series_index"), "02.0f", " #", " - ")))'}{title}