That would be 'normal' I think. I have seen various 'gos' at it, including FDP end at chox+15 and Duty end at 15 after that, but the 'norm' (if there is one) is as described above, and the figure of '30' mins appears almost universally, adjusted as above for unexpected delay in 'clearing up' eg c/crew getting +45 mins to allow for entering sales/bar stuff.
Thus FDP ends on chox, 'rest' begins chox+30 (and is based on 'duty'). -chox'. Discretion, therefore (AKA extension of FDP) is normally related to on-chox.