his been in county four months and its his first time in state prision
More information is needed in order to give you an accurate answer - What is the current charge for which he is going to prison? Does he have a previous strike?
Criminal Defense Attorney
Two ways to arrive at a ROUGH calculation: (1) take the 24 months, subtract 4 months, multiply by 85%; (2) take the 24 months, multiply by 85%, subtract actual time served in county jail.
By the way, CDC will calculate his release date.