For a scholarly attempt at finding a principleddifference between -ic and-ical,
see Stefan Th. Gries's "A corpus-linguisticanalysis of English -ic vs -ical adjectives".
Gries compares, in Table 1, the meanings of the -ic and the -ical forms based on the specificroots polit-, econom-, histor-, class-, lyr-, mag-, com-, electr-, analyt-, logist-, geometr-, numer-, symmetr-, graph-, and problemat-.
He also presents,in Table 2, a set of general principles that partially differentiatethe two suffixes.