Brihat Parashara Hora Shastra
दशाब्दा भुक्तभागघ्ना त्रिंशता विहृताह् फलम् । भुक्तं वर्षादिकं ज्ञेयं भोग्यं मानाद् विशोधितम् ॥ १९०॥
For finding the balance of Dasha at birth, multiply the expired degree etc. by the Dasha years of the first Dasha Rāśi and divide it by 30. The years etc. so arrived at may be deducted from the Dasha years. The result will indicate the balance of Dasha at birth in years, months etc.
english translation
dazAbdA bhuktabhAgaghnA triMzatA vihRtAh phalam । bhuktaM varSAdikaM jJeyaM bhogyaM mAnAd vizodhitam ॥ 190॥
hk transliteration by Sanscript