It'll also depend on how long you were on standby before you were called.
If you were called out from standby to conduct a duty before completing six ours standby duty then the total duty period allowed is the sum of the time spent on standby and the FDP allowed.
If you were called out from standby to conduct a duty after completing 6 hours or more standby duty, then the total duty period allowed is the sum of all the time spent on standby and the allowable FDP reduced by the amount of standby worked in excess of 6 hours.
CAP371