The sheriff's office or county jail where you will serve the time will most likely be able to answer any questions as to what kind of credit they are giving inmates who are serving jail time. Each county jail has different policies based on overcrowding and other issues, so your best bet is to directly confirm with Ft. Bend County in this case. You probably won't get this in writing in the criminal judgment against you, but it wouldn't hurt to ask either. Good luck to you.
My comments are not intended to establish an attorney-client relationship, are not confidential, and are not intended to constitute legal advice. Proper legal advice can only be given by an attorney who agrees to represent you, who reviews the facts of your specific case, who does not have a conflict of interest preventing the representation, and who is licensed to practice in the appropriate jurisdiction where the legal issue may be filed or in the state where the law applies.
The 2-for-1 ratio or whatever the county's rule is on good time is up to the sheriff of that county. This will not appear on a judgment and will not be ordered by the judge, so you cannot get it in writing from anyone other than your attorney. Unfortunately, since your attorney does not likely run the jail, he or she will probably not want to put a guarantee in writing either. However, in my experience, I have never heard of someone not get the going good time rate for a county unless they cause problems in the jail/commit new offenses in the jail No good behavior = no good time credit.
The law is clear on credit for time served - if you served 1 day in jail already, then you have 1 day of credit.
The law is also clear on jail "good time credit". If the sheriff determines to give 2 days of credit for every 1 served, then s/he can. In Ft. Bend, as in Harris County, they routinely give to all inmates 2 for 1 credit. You cannot get anything in writing unless you get something from the Ft. Bend County Sheriff's department, but even then, they will tell you that it is not a given and you must not get into trouble while in jail or it can be taken away from you.