Kobo supports a series index with decimals.
My notation 4.07 means 4-7
My notation 3.5 is something between 3 and 4
There are a couple of cases this plan falls apart. 8-10 (8.1), Gaps 2,4,6
BTW When I see 8.1 and no other 8.n, I infer it is a 10
What is more important: YOU just have
a plan and you use it consistently.
BTW I avoid Series numbers in the {Title} when ever possible.
When a series is released as a group of omnibus books where the title words only differences IS the Series index numbers, I have to keep those in some form to break the Name tie. Series episodes might be Ep##, (the {series_index} I enter is 0.n). Vol # in a title is left, bit {series_index} is the actual value of the stories.
Of course, This is a
Personal Use plan. There is no Official 'series' rule