Origin of values for "security margin"? It seems that the acceptable "security margin" for ciphers is set to be between 25% and 30% as a target by designers, where this number represents the number of rounds that remain "unbroken" for a certain worse-case attack.   It is all too eerily empirical from a field that dislikes that sort of thing considering that the security margin tends to decrease over time as attacks improve.

Is there a historical reason for picking this arbitrary number or is there just a consensus that better than 25% is good enough?
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%.