I set my guild at 500, and none have much raiding experience.
The reality is that the dps is very forgiving and they have it set up to where every person you have past 6 dps is expected to contribute progressively less. I would make sure your tanks are above 520, but everyone else should be fine above 500 if they are fairly decent players. Also, the first 4 bosses are very easy.
The real problem is if your PUG changes members every week.
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Blizzard never had a requirement for Flex.. it is the same as normal mode raids.
Are you maybe confusing heroic scenerios??
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That was my thinking as well. Except for the tanks, there are really no big dps races on the first 3 bosses. It is more about avoiding avoidable damage and smart dps. The devs also said that the dps expected from each individual players is lower the more people you add. So if you add a 14th dps, he won´t cause the boss to increase in health as much as your 7th dps did. (hope that makes sense).
I am not that worried about it. I am telling my guild 500, but I know the tanks and healers are much higher than that, so I will let some dps slip in low and see how it goes.
Another thing to remember, the first boss is very easy. What you may do.. Flex the first two bosses, then if you have trouble on 3 or 4, maybe go and do normal with your 10 best and get some gear and go back and do flex 3 and 4 the next night.