Surya Siddhanta
मध्यग्रहणतश् चोर्ध्वम् इष्टनाडीर् विशोधयेत्। स्थित्यर्धान् मौक्षिकाच् छेषम् प्राग्वच् च्छेषम् तु मौक्षिके॥
If it be required to know the Koṭi &c. at a given time after the middle of the eclipse, subtract the Ghatikas (between the given time and the end of the eclipse) from the second Sthityardha; from the remainder find the Koṭi &c. as mentioned before. The obscured part found from the second Sthityardha is the portion of the disc yet in obscurity.
english translation
madhyagrahaNataz cordhvam iSTanADIr vizodhayet। sthityardhAn maukSikAc cheSam prAgvac ccheSam tu maukSike॥
hk transliteration by Sanscript