Brihat Parashara Hora Shastra
प्रकृयतो विंशतिश्चाब्दा आयुःपिण्डाः प्रकीर्तिताः । नीचगेष्वेतदर्धञ्च ज्ञेयं मध्येऽनुपाततः ॥ ७॥
And, if the Grahas are in between exaltation and debilitation, the rule of three process should be used. Deduct the actual position of the Grah from its deep exaltation point.
english translation
prakRyato viMzatizcAbdA AyuHpiNDAH prakIrtitAH । nIcageSvetadardhaJca jJeyaM madhye'nupAtataH ॥ 7॥
hk transliteration by Sanscript