You may want to look at note 2 here in the regs:
531 .3.2 Unwanted tripping
Residual current protective devices shall be selected and erected such as to limit the risk of unwanted tripping. The
following shall be considered:
(i) subdivision ofcircuits with individual associated RCDs. RCDs shall be selected and the circuits subdivided in such a way that any earth leakage current likely to occur during normal operation of the connected load will not cause unwanted tripping of the device. See also Section 314
(ii) in order to avoid unwanted tripping by protective conductor currents and/or earth leakage currents, the accumulation ofsuch currents downstream ofthe RCD shall be not more than 30 % ofthe rated residual operating current
NOTE1: ThiswillalsoallowabetterselectionofthetypeofRCDsaccordingtothenatureofthecircuitortheload.
NOTE 2: RCDs may operate at any value of residual current in excess of50 %of the rated residual current. (iii) use ofshort time-delayed RCDs, provided the applicable requirements ofChapter 41 are met
A design figure of 30% makes sense I suppose