I used to use the method like higgs...
Then some nasty authors

went and made their series longer than 9 books
Not perfect (because of Nasty Publishers) who then issue omnibus editions that skip around,
and calibre drops any trailing 0's on others,
I infer their meaning, while keeping the ability to properly sort by index (the integer being the primary value)
1.03 (first dot 2 digit last), 12.15, 22.23 for ranges
I also use a Tag Omnibus Edition)
n.5 for in between stories (this can be confusing when .5 is not
looking out of place
10 for singles
Splitting works best (and helps performance on many mobile devices)