Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 2 · Signs, Planets, Birth
जातकम् १
पिंडे निसर्गे रवोच्चे नो ग्रहः षट्भाल्पको यदा ।चक्रशुद्धस्तदा ग्राह्येस्यांशा आयुषि संमताः ॥ १२२ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 55, verse 122.
For pinda (total figure) and nisarga (natural lifespan), take the planet's high point from the planet. If that is less than six signs, take it from the full circle. Its degrees are then counted for the life.
Simple meaning: A planet's full years count by how far it stands from its low point.
अंशोनाः शत्रुभे कार्या ग्रहं वक्रगतिं विना ।
A planet's years are reduced when it is in an enemy sign or set in the Sun's light.
हानिद्वयेऽधिकाः कार्या यदा क्रूरस्तनौ तदा ।
When two cuts apply, only the bigger one is made. A cruel planet rising also cuts the years.
भगणांशैर्लब्धहीनास्तेषां कार्या विचक्षणैः ।
The rising planet cuts the years by a share. A bad one cuts the whole, a good one half.