Brihat Parashara Hora Shastra
दृश्याद् विशोध्य द्रष्टारं षड्राशिभ्योऽधिकान्तरम् । दिगभ्यः संशोध्य तद्भागा द्विभक्ता दृक् स्फुटा भवेत् ॥ ६॥
Evaluation of the Drishtis of the Grahas. Deduct the longitude of the Grah (or Bhava), that receives a Drishti, from that of the Grah, which gives the Drishti. If the sum exceeds six Rāśis, deduct the sum again from 10 Rāśis. Convert the latter sum into degrees and divide by two.
english translation
dRzyAd vizodhya draSTAraM SaDrAzibhyo'dhikAntaram । digabhyaH saMzodhya tadbhAgA dvibhaktA dRk sphuTA bhavet ॥ 6॥
hk transliteration by Sanscript