Brihat Parashara Hora Shastra
लब्धांशास्तु यथाप्राप्तखण्डघ्नास्त्रिंशतोद्धृताः । लब्धवर्षादिभिर्हीनं प्राप्तायुः प्रस्फुटं भवेत् ॥ ४६॥
The latest product should be multiplied by the number of basic years and divided by 30. This will yield the net longevity.
english translation
labdhAMzAstu yathAprAptakhaNDaghnAstriMzatoddhRtAH । labdhavarSAdibhirhInaM prAptAyuH prasphuTaM bhavet ॥ 46॥
hk transliteration by Sanscript