Brihat Parashara Hora Shastra
चक्राद् विशोध्य तद्भागास्त्रिभिर्भक्ताश्च दिग्बलम् । इष्टाधटि निशीथात्तन्नतं त्रिंशच्च्युतं नतम् ॥ ८॥
Kaal Bal (up to Sloka 17). Firstly Nathonnata Bal. Find out the difference between midnight and the apparent birth time, which is called Unnata. Deduct Unnata from 30 Ghatis to obtain Nata.
english translation
cakrAd vizodhya tadbhAgAstribhirbhaktAzca digbalam । iSTAdhaTi nizIthAttannataM triMzaccyutaM natam ॥ 8॥
hk transliteration by Sanscript