I cannot determine if the punishment provided was fair, but I must assume it to be within the legal safeguards of AR 27-10 and the UCMJ. This process is further protected when the Article 15 is reviewed by JAG for legal compliance.
With that stated, a Commander can consider daily performance and attitude to assist in determining appropriate punishment. Also keep in mind if you have done several things and are pending an Article 15 they must all be rolled into one Article 15 for punishment. So it is possible another offense or action was considered with regard to your punishment.
If you believe you were punished excessively, I would encourage you to file for an appeal. If you prepare a good case by removing emotion, stating only facts, remaining professional, and having a realistic request for reduction of punishment you may be able to have the punishment modified. I doubt you will be successful in having the Article 15 thrown out but a modification may be appropriate. It would be wise to seek guidance from your JAG office to decide if an appeal should be attempted. If you have already refused the appeal you can still change your mind as long as the time frame is reasonable. Again, see JAG with regard to this issue.
I have provided some extracts of AR 27-10 for your review.
3–19. Rules and limitations
a. Whether to impose punishment and the nature of the punishment are the sole decisions of the imposing commander. However, commanders are encouraged to consult with their NCOs on the appropriate type, duration, and limits of punishment to be imposed. Additionally, as NCOs are often in the best position to observe a Soldier undergoing punishment and evaluate daily performance and attitude, their views on clemency should be given careful consideration.
3–10. Double punishment prohibited
When nonjudicial punishment has been imposed for an offense, punishment may not again be imposed for the same offense under Article 15. Once nonjudicial punishment has been imposed, it may not be increased, upon appeal or otherwise. When a commander determines that nonjudicial punishment is appropriate for a particular service member, all known offenses determined to be appropriate for disposition by nonjudicial punishment and ready to be considered at that time, including all offenses arising from a single incident or course of conduct, will ordinarily be considered together and not made the basis for multiple punishments. This provision does not restrict the commander’s right to prefer court-martial charges for a non-minor offense previously punished under the provisions of Article 15.