440 CR 1/4 creatures
220 CR 1/2
110 CR 1
55 CR 2
45 CR 3
35 CR 4
25 CR 5
15 CR 6
and 8 CR 7
edit: As for modifiers...there's a hell of a lot of them. Reputation of cruelty gives a +2, success in combat (and what PC doesn't have that reputation?) gives +1, as does magical power or working with demons/devils. A reputation of losing battles, on the other hand, gives -1, as does one for restraining looting/pillaging, and a reputation of mercy gives -2.
Previous events can also influence your tyrant rating - victory over an enemy army gives a +4, the character being defeated in personal combat gives -6, being defeated by an enemy army gives -8, an unsuccessful coup (I would assume on your part) gives -2, looting a village gives +4, looting a city gives +6, defeating a tribal leader in personal combat gives +1, you get +2 for your horde actively campaigning and -4 if it's inactive.
On the whole, I'd say the modifiers aren't terribly well thought out. For one thing, there aren't really any states inbetween 'actively campaigning' and 'inactive,' so you'd almost never use the base value...