Origin of values for "security margin"?
There is not much reason behind this particular number, but I think that the AES cipher stands as an example. By the time of its selection as AES, Rijndael-128 had 7 out of its 10 rounds broken, though for the 7-round attack it was not clear then if it is better than the exhaustive search. So the security margin was about 30% that time. Now, as the attacks have de-facto advanced by 1 round only (the biclique attacks have only a small constant advantage factor), it is understood that the security margin was well chosen and it made sense to have it 30%.