There is no formula to the art of negotiation and bargaining. Every case is within itself as intricate as anything in human relations. There is no formula for give and take. Otherwise, any one "side" could just think one step turther ahead and win, as in chess.
You have to employ an attorney. Consider how different is for you to say that you are a hardworking kid who just made a mistake or for me to say it. A huge percentage of cases settle out through bargaining. It doesn't make good TV, so there are no "NY Plea Bargain" shows. But, that is what lawyers do, that is their real skill. But, it the offer is bad, then their real skill is winning at trial.
Get an attorney, the best you can afford.
We do not have a client/attorney relationship until you make an appointment, we discuss your case face to face, I accept a retainer, and we explictly agree to enter into representation.
Its a personal decision on the part of the prosecutor or commonwealth attorney. They may be insulted you turned down the plea and want to stick it to you or they may not care at all. A local attorney should know that attitudes of the prosecutors and judges. If you are 18 you definitely need a private attorney or state provided attorney public defender through Dept. of Public Advocacy.
In some counties in KY, after the first plea bargain is offered and rejected the prosecutor will not make another offer. This is not always the case however. The most important thing for you to do before deciding to accept a plea offer is discuss all of your options with local, competent counsel.
A defense attorney who practices in you area will be able to advise you of the reasonableness of the plea bargain.
This is general legal advice. You should speak with an attorney about your particular case.