Brihat Parashara Hora Shastra
गततारास्त्रिभिर्भक्ताः शेषं चैव चतुर्गुणम् । वर्तमानपदेनाढ्यं राशीनामंशको भवेत् ॥ ८८॥
The number of Ashvini etc., whichever may be the past Nakshatras, should be divided by 3. Thereafter the remainder should be multiplied by 4. To the figure so made available the Pad of the present Nakshatr should be added. The product will be the Navāńś from Mesh onwards.
english translation
gatatArAstribhirbhaktAH zeSaM caiva caturguNam । vartamAnapadenADhyaM rAzInAmaMzako bhavet ॥ 88॥
hk transliteration by Sanscript