Need help on a Save to Disk template

I'm trying to come up with a Save-To-Disk template to allow me to export my books. I've figured out some of what I need to do, but I'm stuck on the final piece. Specifically, I need to remove the extra underscores that get generated from the template when a book has no subseries and/or series.

I have the following custom columns:

Division (Major classification for sorting libraries)
Genre (Fiction, Reference, Science Fiction, etc)
AuthorLookup (For dealing with multiple authors, etc)
Subseries (A series-like field used when there's more than 1 level to the series information. If a book has a subseries, it will always have a series as well)

Currently, my template looks like this:

In plain English, I always want the Division, Genre, and AuthorLookup as nested folders. Inside the AuthorLookup, I want just the book title if there's no series. If there's a series, I want another subfolder for the series, with each book in the series starting with its index (padded to 3 chars) followed by an underscore. After that, if there's a subseries I want the subseries name and index (padded to 3 chars) followed by another underscore. Finally, I want the book title.

Some examples of what I want:

If I just use the template above, I get:

As you can see, in cases where there's no series, I get 2 underscores in front of the book title (I think one of these is converted from an empty folder name).
In cases where there's a series but no subseries, I get 2 underscores instead of 1 between the series index and the book title.

I've figured out prefixes and suffixes, and they help some, but I can't use them with padded indices (I get an error when trying to Apply). So how do I strip out the extra underscores when there's no subseries (or series)?