Brihat Parashara Hora Shastra
ततस्तद्यातभोगाध्यां भुक्तं भोग्यं च साधयेत् । विंशोत्तरीवदेवात्र ततस्तत्फलमादिशेत् ॥ २२॥
Thus the expired portion of the Dasha is calculated, according to the method, followed for Vimshottari Dasha, by multiplying the Bhayat, i.e. the expired period of the stay of Candr in the Janm Nakshatr, by the Dasha portion of the Janm Nakshatr and dividing it by Bhabhog, i.e. the total period of the stay of Candr in the Janm Nakshatr.
english translation
tatastadyAtabhogAdhyAM bhuktaM bhogyaM ca sAdhayet । viMzottarIvadevAtra tatastatphalamAdizet ॥ 22॥
hk transliteration by Sanscript