1. 10.82 years = 10 years, 9 months, 25.6 days.
2. Credit for time served should be applied at sentencing so that the time left to serve is the total sentence minus the time already served (that is the "credit" you receive). In this case it would be difficult for me to calculate a release date without knowing the date of incarceration.
3. As far as the 85% goes, it is my understanding that "good and gain time," as it is called, is determined by the department of corrections, not by the court or the sheriff. That means they can dispense or withhold it as they see fit. They can even increase it. There has been a rumor circulating that they planned to increase the time off from 15% to 25% due to overcrowding and budget issues, for example, but it is a rumor only, as far as I know.
You need to use his incarceration date, take away time served credit from the total sentence, then project what's left forward to derive a release date. Web sites such as Wolfram-alpha are helpful for doing these date calculations. Good luck.