Is there a publication of feedback about promotion board results?

I can tell you the litmus test for qualification for promotion is fluid. A good friend of mine was 4:5 ACOM OERs, had all school requirements and multitude of assignments in various Echelons and Joint yet still was not picked up. So you are not the only one wondering but which is evident to the fact that you need to be ready for you are not selected for promotion just as much as you are prepared for promotion.

It changes every board. The best thing to do is just do your best, don't worry about the ratings or box checks and hope the Army rewards you for your hard work.

I don't think there can ever be a statement that you need two ACOMs to promote. That may be the result of the overall but not a gate. It's too subjective.

If you wait long enough and are lucky, the slides that HRC has an operational research type produce that show trends for your particular board and that are used to brief Army senior leaders will "leak" from HRC and provide some level,of transparency. If you're really lucky, HRC will actually release some (but not all) of the data and documents related to the board. Note, though, that even if this data comes out, it's not related to how the board actually functions or the instructions given to the board. These sorts of slides are simply a statistician's attempt to find trends in data.

Senior Army leaders are interested in the type of question you are asking. I just spent a week escorting a general, and these HR types of questions came up quite often. If you get a chance to engage with a senior leader, ask this type of question. As a big institution, the Army's senior leaders are sometimes in an "emperor's new clothes" situation where the sycophants surrounding them are filtering feedback and preventing a clear understanding of the situation, in some cases. This won't change unless senior leaders are asked tough questions and made aware of the issues the force sees.

Ask for transparency in the process. The Army should be releasing the instructions to the board for every board, the order of merit list, and all other pertinent documentation.m the Army hides behind privacy laws to keep this data hidden.

Ask for an honest Army messaging campaign about the drawdown. The current messaging campaign is not ideal, nor is it honest. We aren't returning to historic promotion rates, but rather fully abiding by DOPMA (which isn't required) and seeing the lowest promotion rates in history.

Ask for a re-evaluation of the process. Congress forced some of the drawdown on the Army, the Army is choosing to do some parts of the drawdown. Every dollar that is spent on equipment or benefits or retiring a general at four stars vs. three or less when he's a convicted felon is the Army prioritizing something other than keeping the best people in the Army.

I will not echo what the others have said, as MAJ Michael Pauling hit the nail on the head with being prepared.
I will tell you what several of my bosses both past and present have told me from their time on promotion boards. The board wants to see a heartbeat, meaning when you do get a COM or two, does your performance go up or are you a flatliner at a certain standard? Yes they look at only the last 5 OERs as each person's file generally only gets 2-3 minutes in front of the board on the first look. With that they will generally only look at the SR comments, no disrespect to your Rater but the guys on the boards are likely to be peers with SRs so that is who they are looking at for advice on you. Your DA Photo is on one screen and your OERs are on the other.
I know it was a lot of rambling but that is what I can remember.

As stated previously boards are very fluid. The key is to do your best. Do the KD jobs and keep your nose clean.

I had a good friend and retired AG officer once tell me, "records are either perfect or they are not". If some one appears overweight on the photo or bad photo in general, a non left justified box check, or referred OER, Army career is likely to be over.

For the the rest of the files, who knows. Every officer at the board reads the instructions to the board. In the end they cast their vote, and they are human.

There is no formula for ACOMs, a specific amount of deployments or BSMs/MSMs are needed. But as I mentioned above there are things that make a file stand out for the wrong reasons.

The best we can do is like MAJ Michael Pauling said, to be prepared for promotion and also for separation/retirement.

I don't know if this is what you are looking for, sir, but here's a PCC for your board packet and some basic observations from Army G-1:
https://www.hrc.army.mil/TAGD/Preparing%20Your%20File%20for%20Promotion%20or%20Selection%20A%20Pre-Combat%20Checklist

Made it! Plus resident-course!