Every case is fact specific. In Federal criminal matters the District Court is required to consult the Federal Sentencing Guidelines. The Sentencing Guidelines take into consideration the Defendant's Criminal History, and Offense Variables relating to the charged offense. Once these issues have been determined, the guidelines would provide an advisory sentencing range. provided to the District Court. A US District Court Judge has very broad discretion in determining an appropriate sentence....
My experience is that your letter will not be read by the Judge and will be intercepted by the Court staff. The Judge's assistant will write you a polite letter explaining this procedure to you. I would encourage you to seek legal counsel to ensure the facts are known to the Court.