The statutory requirement is DOPMA (Defense Officer Personnel Management Act of 1980 (Public Law 96-513). Promotion opportunity is supposed to be consistent over a 5 year period. The CBO did an interesting (if a little nerdy) study on the last couple drawdowns and how the services handled them - it helps explain the process if you are really interested. This all goes back to the "up or out" argument and the effect it has on how the military does personnel business.
NPC really does calculate opportunity the "correct" way, but that number doesn't mean anything to the guy a few years out trying to figure out what his chances really are since there is no way to know if your in-zone year is going to have a board that picks up a lot of folks above zone (though it seems boards have figured out that below zone picks just fornicate the system all around). Another problem to me is that they don't provide meaningful board statistics like breaking out 1310/1320 promotion rates by community. Go back in the archives if they are still up and you can see that what community you are in really does matter (though at the O-5 level that is partly due to the way the squadron's are organized and the number of DH's in competition).
NPC really does calculate opportunity the "correct" way, but that number doesn't mean anything to the guy a few years out trying to figure out what his chances really are since there is no way to know if your in-zone year is going to have a board that picks up a lot of folks above zone (though it seems boards have figured out that below zone picks just fornicate the system all around). Another problem to me is that they don't provide meaningful board statistics like breaking out 1310/1320 promotion rates by community. Go back in the archives if they are still up and you can see that what community you are in really does matter (though at the O-5 level that is partly due to the way the squadron's are organized and the number of DH's in competition).