I classified Haki into various levels based on Blaize's attribute points.

[10 - 20] - Basic

[20 - 30] - Intermediate

[30 - 40] - Advanced

[40 - 50] - Yonko Commander

[50 - 60] - Admiral level

[60 - 70] - Yonko [Four Emperors, Fleet Admiral]

[70 - 80] - Pirate King [Whitebeard, Roger, and Garp]

[80 - 90] - ????

It can also be used as a reference for one's physical strength and Devil fruit. A person's strength differs when both his strength and devil fruit is taken into account.

For example, Kaido, even if his Haki is only at the level of Yonko, combined with his mighty physique and powerful devil fruit he can fight someone above his level.

Another example is Kizaru, he's good at using his Devil fruit while slightly weaker in terms of Haki.

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This is not definite. The above levels may change as the story progress. And, if you have any better ideas please comment below...