Attached you will find a Good Time Table PDF by Daniel Hesler, a staff attorney of the Federal Defender Program in Chicago. Although it is not an official BOP document, and shouldn’t be relied as such, the table calculations seem to be accurate and a handy tool.
You can also do your own calculation, and it should be accurate, by simply multiplying the number of months in a sentence by 4.5. That gives you the number of good time days possible. Forget about 85%. You can tell your clients; “Good time is 4.5 days for each month you are sentenced to, as long as you behave yourself.”
Again, if you are wondering why, it’s this; the statute says good time is 54 days a year. 54 divided by 12 is 4.5 days per month.