Brihat Parashara Hora Shastra
दशामनां भयातघ्नं भभोगेन हृतं फलम् । दशाया भुक्तवर्षाद्य भोग्यं मानाद् विशोधितम् ॥ १६॥
To find out the remainder of the Dasha, operating at the time of birth, first find out the expired portion of the Dasha of the concerned Grah. This is done, as follows. Multiply the Dasha period of the Grah concerned by the period of the stay of Candr in Janm Nakshatr, that has expired and divide that amount by the total period of the stay of Candr in that Nakshatr. The figure in years, months etc. so arrived at will be the expired period of the Dasha. If this figure is deducted from the total period of the Dasha, we will get the balance of Dasha at the time of birth.
english translation
dazAmanAM bhayAtaghnaM bhabhogena hRtaM phalam । dazAyA bhuktavarSAdya bhogyaM mAnAd vizodhitam ॥ 16॥
hk transliteration by Sanscript