I wondered the same thing before I nominated myself.
You can hover your mouse over the existing nominations and see what each score component is...It looks like the n/40 score is based on 4 things:
- 1 point per 1k of reputation, up to 20k rep (so 20 points max)
- Up to 8 points total for each review queue badge (Civic Duty, Deputy, Reviewer, Steward).
- Up to 6 points for each editing badge (Explainer, Organizer, Strunk & White, Tag Editor)
- Up to 6 points for each participation badge (Constituent, Enthusiast, Quorum, Yearling)
But, as Mapperz points out, there's not a minimum score to nominate yourself other than the minimum reputation requirement (300).