How is the starting_target or minimum_target for a mining pool decided?

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I am working on a understanding all the possible solutions currently available for payout mechanism in case of pooled mining and i have one question which is answered vaguely but not concretely i.e. the factors to be considered while deciding starting_target for a mining pool which eventually be adjusted according to pool specific difficulty adjustment logic and decaying function for score-based pools such as pplns,geometric etc.