How exactly did Hawking compute that black hole entropy is 1/4 that of a planck area and concluded about the holographic principle where information of a volume is located on the area of black hole? And if there was no holographic principle, how big should entropy of the black hole be with reference to planck area (would it be say 1/8 or twice that of planck area for example)? How is this computed? What observational evidence is there for these? Do all observations still obey the holographic principle?