Hello,
Swords Oakeshott Type XIII have a fuller about half blade.
I read somewhere, that can be at most 5 / 8 of the blade.
Now I can not remember what book or forum.
A confirmation with a bibliographic source would be appreciated.
Thanks.
Ciao Maurizio.

This is incorrect. There are no hard-fast rules for Oakeshott's typology. There are only averages, norms, typical cases, etc. And even then, the typology is designed with the intention only of being a means by which to have a conversation. One could theoretically say, "this sword is an Oakeshott Type XV with a small central fuller" and that's a perfectly valid conversation to have despite the fact that a fullered doesn't fit the definition of a Type XV blade.

The typology is an attempt to group swords into categories for discussion. It isn't a system in which swords were made to fit. You may be misunderstanding the intention of the typology system.