If an inmate was arrested on 9/18/2020 and took a plea deal on 2/28/2021 for 5 years would he get credit from the booking date and how much time would he have to serve with good time because his release date is showing 2025?

He should get credit for every day spent incarcerated. The booking arrest date is the first day, even if they were booked and released, they get a full day's credit. Five years is 60 months, good time is 15% so the actual length would be 51 months not counting some for halfway house (if offered)