I am trying to get a understanding of calculating time?

I'll assume you mean how much actual time would be served on a particular sentence... it starts with "good time credits" that are applied to the sentence calculator a the beginning of the sentence. An inmate can only lose that good time by having behavior issues or breaking numerous rules. In federal you get 15%, state prison ranges from 15-30% good time. The difference with the state is that they're also dealing with a parole system too that can release an offender having done only one-third of their sentence.