For the Jails.... and also prisons?
It used to be 65% and there has been talk it may go back
Is there a place this information is given- an official website?
If you are asking about Florida State prison sentences then the answer is that if you behave in prison, and if you are not serving a minimum-mandatory sentence, then you will serve 85%.
If you are asking about any State other than Florida then you should repost your question with a different location.
Unlike the FL-DOC, local jails operate on municipal rules and are not standardized throughout the State. You can find out the gain time for any particular institution (state, local or even federal) by doing a Google search for the specific facility and either reading the information online or contacting them directly and asking.
For example, the Ft. Pierce jail's website is: http://www.stluciesheriff.com/disclaimer.php
I hope that I have been helpful in answering your question.
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The answer is found in the Florida Statues for Florida and each state has their own laws as well as the Federal system. For the rest of the answer, see Mr. Haber's post which is spot on correct, as usual.
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It is 85% in FL. You can go to the FL department of corrections website to find all sorts of useful information, even a rather telling history of corrections in FL.
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