[Adjustment for dividend]

Hey guys. Just wandering how IG calculates for adjustment for dividends. I understand it is to combat against people shorting at ex-dividends when prices go down. But, i shorted a share a day before ex-dividend day and the bank statements said that the adjustment for dividend was at 9:55, the next day on ex-dividends, which is 5 minutes before the markets open. Is this a estimate from previous ex-dividend days or by the dividend amount?

Thanks IG community 

[Adjustment for dividend]

Hey guys. Just wandering how IG calculates for adjustment for dividends. I understand it is to combat against people shorting at ex-dividends when prices go down. But, i shorted a share a day before ex-dividend day and the bank statements said that the adjustment for dividend was at 9:55 which is 5 minutes before the markets open. Is this a estimate from previous ex-dividend days or by the dividend? 

Thanks IG community