Brihat Parashara Hora Shastra
योगार्धे च दशामानं द्वयोर्योगार्धसम्मितम् । लग्नसप्तमप्राण्यादिर्दशेयं च प्रवर्तते ॥ १७४॥
The spans of Dashas of the Rāśis in the Yogardha Dasha system are half of the total of the spans of Char and Sthir Dashas. The Dasha will commence from the Rāśi of Lagn, or Yuvati, whichever is stronger. The order of the Dashas of the 12 Rāśis will be counted onwards, if the opening Dasha Rāśi is an odd one. If it be an even Rāśi, the Dashas will be in reverse order.
english translation
yogArdhe ca dazAmAnaM dvayoryogArdhasammitam । lagnasaptamaprANyAdirdazeyaM ca pravartate ॥ 174॥
hk transliteration by Sanscript