The calculation of your credits is dependent on various factors, such as your criminal record, whether you have any prior strikes, whether you are being charged with felonies or misdemeanors, whether you are being sentenced to state prison or county jail, etc. You haven't provided many details here to determine what credits you are owed. With that said, however, if you are confused about your credits, but you want to resolve your case, make sure you speak to your attorneys about the credits prior to sentencing and have your lawyer confirm your eligibility for the percentage that you would receive in good-time/work-time credits. It is totally appropriate for you, at the time of sentencing, to inquire from the Judge personally about what type of credits you will be receiving. In the event you are then given credits different from which you were told, you would have preserved a record as to what your understanding so that you could most definitely return to court and challenge your plea deal and/or move to vacate your plea deal if it does not comport with how much credits should have been owed to you. If a Judge is wrong, the probation department and/or the Dept of Corrections and/or the Sheriffs at the County Jail (whichever applies), could send you back to court for a recalculation of credits. Don't be shy to inquire until you are clear that you are given the right answers. There is no such thing as a stupid question when it comes to calculation of credits and having assurances that it is accurate.
How long is the offer? Try to get a "credit for time served" offer. The judge will calculate your credits on the bench and you'll be released soon after. Mr. Feyzjou's answer is excellent and comprehensive. You definitely need to talk this out with your attorney.
You should speak with your attorney about this, they should be able to provide you with the best answer you'd trust. As a general rule though, don't discount what a judge says because of another inmate told you; judges generally know the law better than other inmates.
Nothing said here shall be construed as legal advice. I can not effectively advise about your case without knowing all the facts. Additionally, my response does not create an attorney-client relationship. If you require further assistance, please contact an attorney privately.
I echo Mr. Wen's comment that you should not listen to what inmates tell you about the law. If they knew so much they wouldn't be in jail or prison. The person who put them there, i.e., the judge is a much better source of information. As to what the inmate told you everybody with a county jail sentence now receives day for day credits regardless of background or priors. Those credits will be recalculated if you are sentenced to prison but as long as your sentence is a county jail sentence you get day for day credits.