Sir, in fact, there is no parameter which can predict with 100%.The train, season, weekday / Holiday(s)/festivals, disruption or influence of nature / political parties etc. are the cause which cause variance to upgradation from RAC to CNF or WL to RAC. Apart from all these, bulk booking / cancellation, coach removal / induction, alternative train withdrawal / insertion are further affecting the upgradation together with normal up/down by routine passengers.
Hence, the prediction in a few cases differ.
Hope, you are more clear about.