Brihat Parashara Hora Shastra
कारकादष्टमेशो वा ब्रह्माऽप्यष्टभावगः । शनौ पाते च ब्रह्मत्त्वे ब्रह्मा तत्षष्ठखेचरः ॥ १७२॥
The Lord of Randhr in Randhr is also accepted, as Brahm Grah. If Śani, or Rahu/Ketu obtain Brahmatva (qualifications of Brahm Grah), they become Brahm Grah. If a number of Grahas obtain Brahmatva, the one with the largest number of degrees would become Brahm Grah.
english translation
kArakAdaSTamezo vA brahmA'pyaSTabhAvagaH । zanau pAte ca brahmattve brahmA tatSaSThakhecaraH ॥ 172॥
hk transliteration by Sanscript