Outsiders view here but I would think you would take all those who have deployed and screen that list for those you view as capable of being promoted. Some will just not be cut out for promotion some will need more time to be ready. But you take as many of those that have deployed that are ready first.
After that you screen those that havent deployed and take the best of those to fill out rest that you need.
People that have deployed have earned that right to be considerd first what is point of military asking society to show respect towards vetrans if military itself doesnt.