I need to find out the correct naming convention for non dialogue sentances?

The character's first name is Elektra, however the character prefers the shortened version "Ellie". When writing the sentences involving this character, which name is the correct version?





Example: 'Elektra started the car.'


'Ellie started the car.'


Some novels (especially Phillip K Dick and Stephen King) have non dialogue sentences where the character is referred to by the shortened version of there name such "Rick" instead of Richard (Do Androids Dream of Electric Sheep). But I don't know if this method is acceptable to publishers.

I need to find out the correct naming convention for non dialogue sentances?
When narrating, Elektra is correct. When other characters interact with her, either name can be used. This keeps the reader inside the dialogue when it is personalized.
Reply:This question gives me a headache, thanks alot.