I just use a 0
IMHO that is less jarring than 3 9's
To me, a Zero means it has no place in the timeline.
I use fractionals for stories (usually shorts) the take place between the major stories.
I used to use [3.45] to indicate the books 3, 4, 5, in an Omnibus until a few publishers broke that by skipping books, or adding a 4th
Now I use 10.12 (implied 11). 10.5 is a short.
Something will always break
Another choice is to use EPUBsplit plugin and break them apart (I leave a note Split_from: xyz)