Brihat Parashara Hora Shastra
यस्मिन् भावे स्थितो मन्दस्तस्य मान्देर्यदन्तरम्। लग्नभाग्यान्तरं योज्यं यच्च राश्यादि जायते॥ २७॥
Add this to the difference between the Lagn Bhava (Madhya, or cusp) and the 9th Bhava (cusp).
english translation
yasmin bhAve sthito mandastasya mAnderyadantaram। lagnabhAgyAntaraM yojyaM yacca rAzyAdi jAyate॥ 27॥
hk transliteration by Sanscript