Hi Kamel,
not sure where you got the 460 kgs from, however i agree with you that holding at around 20 000 ft makes more sense if holding for 20 minutes plus.
Anything more than that, in most cases, would be compensated for with extra fuel by dispatch, or by the operating crew due to an unfavourable TAF/NOTAM etc.
Holding at 1500 ft is very unlikley in most normal situations but it's a good comparison to actual fuel burn across the range.