Brihat Parashara Hora Shastra
सायनार्कस्त्रिभोऽर्कस्य व्यर्केन्दुश्च विधोस्तथा । चेष्टाकेन्द्रं रसाल्पं तच्चक्राच्छोध्यं रसाधिके ॥ ४॥
The sidereal longitude of Sūrya should be deducted from Candr to get Candr‟s Chesht Kendr. If the Chesht Kendr (for any Grah) is in excess of 6 Rāśis, deduct it from 12 Rāśis. Add 1 Rāśi and multiply the degrees etc. by 2, which will indicate the Chesht Rasmi of the Grah.
english translation
sAyanArkastribho'rkasya vyarkenduzca vidhostathA । ceSTAkendraM rasAlpaM taccakrAcchodhyaM rasAdhike ॥ 4॥
hk transliteration by Sanscript