Description - Rank
Civilian - 0
Academy student or some chakra training - 1
Genin - 2
Experienced genin/fresh chuunin - 3
Chuunin - 4
High Chuunin/Special Jounin - 5
Jounin - 6
High Jounin - 7
S-Class - 8
I think filling it out like that puts the top tier as far out of reach as they should be, while making it clear where folks in the chuunin exam stand.
A certain number of milestones + downtown training to internalize experiences.
Nine levels seems like way too many for scale. I'd go with the standard DFAE uses of 5: Mundane, Supernatural, Otherwordly, Legendary, and Godlike. Then just adjust those words to something more like you'd expect in Naruto, and apply divisions thus - because even within the same level of Scale, there's still a lot of variation in capability.
Mundane - Civilians and standard skills that don't fall under the realm of ninja-arts. Academy students start here, and work their way up.
Supernatural - Genin of all sorts, and some Chuunin. Differences are due to actual Skill level than Scale - Students have Mediocre or Average, Genin are Average or Fair and sometimes Good, while Chuunin range from Average to Great
Otherwordly - Jounin territory. Needs years of experience, and loads of power and skill. Most have stats ranging from Average to Good. S-ranked threats and High Jounin can get up to Great, and very rarely more than that under special circumstances (skills at Fantastic and up are ridiculously rare at any Scale, and usually only occur due to Stunts and copious FP usage)
Legendary - The top tier of the top tier. The First, Second, and Third Hokage and the Sannin are at this level, as are folks like Hanzo the Salamander, Pain/Nagato (when he's pulling out all stops with the Rinnegan), and Obito operate at this level. Madara as well, until he starts pulling out his late-game bullshit.
Godlike - Word-enders. Tailed Beasts occupy this tier, but don't have as high skill levels as the Ten Tails. The Sage of Six Paths also occupies this level. Beating them usually entails manipulating Conditions and Aspects and judicious Compels to shut them down. Even so, it's never easy, nor straightforward.
Note, that certain individuals could always have Stunts or abilities that let them treat one or two of their skills as one Scale higher than normal (or allow them to ignore Scale for certain things), which allows you to have some civilians that are capable of dealing with Ninja in certain scenarios (usually politics and diplomacy, which we've seen evidence of with that one Governor back in Iron iirc), or even Tokubetsu Jounin who are Chuunin on all levels, but count as Jounin in certain skills (ie having Stunts that let them have a level higher Scale in stuff like Fists or something).
And even then, differing skill levels adds further distinction when operating in the same Scale. Genin have low skill levels, Chuunin have higher ones. Jounin have a wide range of skills, so a Jounin could have Fists 2 and a Chuunin has Fists 3, but the Jounin still tends to win because the Jounin is Otherworldy while the Chuunin is Supernatural, and thus the Jounin has a level of Scale over the Chuunin.
Also of maybe import: DFAE lists the benefits of Scale as thus:
When applying scale to two opposing forces or individuals, compare each side's level and then apply one of the following benefits to whichever is scaled higher:
- +1 per scale level to their action before the roll
- +2 per scale level to the result after the roll, if the roll succeeds
- 1 free invocation per scale level on a successful advantage after the roll
