What would be your guidance for letter to President of a US Army Reserve Promotion Board for Captain?

F*CK YES. The question is how hard are you are willing to fight, what did you do to warrant a reprimand, and how have your grown since getting one? You don’t have to share/answer any of that except to yourself.

You have the right as an officer to write a letter to the president of the board, while the president may not read it, your briefer will. Giving him/her the entire situation and will allow him/her to brief your summary/OMPF to the board members and their recommendations of where you stand in comparison to the rest of the members of your rank; Promote – highly, medium, lowly recommended…
Write a letter to board addressing the following:

1) What have you current done in your current billet and what was the result of your contributions? Write a clear picture of your recent activities and qualifications. Things that may not be cover in your current period evaluations or more in detail.
2) Address any administrative inaccuracies in your records and provide enclosures to support.
3) Address any bad paperwork you have, situation & circumstances that was warranted, and what have you learned/grown from your mistake? In your case address why you got a reprimand and why the reprimand is getting moved to your restricted file. Attach any and all statements that support your claim. If there was someone in your chain of command that can vouch for your character or situation of this event, have them write you a letter explaining the facts and circumstances.
4) Explain why or why not: Have you completed your Professional Military Education course for your rank? If not, why. IE, pursuit of a civilian advance degree, etc…
5) Tell your story to the board and why you would like to continue serving.

Hopefully, this helps.

Darren, the best guidance is to review the MILPER message as it pertains to your letter. A letter is supposed to address information deemed important to the consideration of your board file—in this scenario it seems appropriate. You cannot, however, add third-party correspondence.

With all the other advise, less is more and more is less. You are not writing a letter to grandma about summer camp and if it is wordy, they will get bored.