>Whereas in the federal system, no minimum is specified before you
>can get out, even though the good time index system probably mathematically
>implies one?
In federal sentencing the United States Code, written by legislators, sets up mandatory minimums. In drug cases the minimums are triggered by quantity and type of drug. This is where the disparities between sentences for cocaine and crack (and now meth) come in. But there are ways to get a sentence that is under the mandatory minimum under the separate system of federal sentencing guidelines. Whatever your sentence, you serve about 85% with good time.
The supreme court decision that declared the guidelines advisory earlier this year has changed things, but not that noticeably yet since the guidelines are all most people in the system know.