Brihat Parashara Hora Shastra
पापदृक्पादहीनं तच्छुभदृक्पादयुक् तथा । बलैक्यं ज्ञेज्यदृक्युक्तमेवं खेटबलं भवेत् ॥ १९॥
Drik Bal. Reduce one fourth of the Drishti Pinda, if a Grah receives malefic Drishtis and add a fourth, if it receives a Drishti from a benefic. Super add the entire Drishti of Budh and Guru to get the net strength of a Grah.
english translation
pApadRkpAdahInaM tacchubhadRkpAdayuk tathA । balaikyaM jJejyadRkyuktamevaM kheTabalaM bhavet ॥ 19॥
hk transliteration by Sanscript