One such method found here is given by Rishi Parasara & is as follows:
Atma Karaka is the planet that has the highest longitude in one’s natal Chart. Atma Karaka in Navamsa is called the “Karakamsa”. The 12th from the Karakamsa determines the “Istha Devata”. The 12th house from the karakamsa is called the “Jivanmuktamsa”. This is the house signifying the emancipation of the soul and we have to examine this house in order to find one’s Ishta Devata. If there is a graha in the Jivanmuktamsa, it signifies the Devata. If there are more grahas than one, we choose the strongest. If there is no graha in the Jivanmuktamsa, we can see whether any Grahas are aspecting the 12th. If no Grahas aspect it, we can take the 12th Lord as Ishta Devata.
Below are the presiding deities for each planet who will be your Ishta Devata as per your Horoscope.
Sun– Vishnu/Rama/Shiva Moon– Krishna/Shiva/Parvati Mars– Hanuman/Subramanya/Sri Narasimha Mer– Vishnu Jup– Vishnu/Sri Vamana/Dattatreya/Sai Baba Ven– Maha Lakshmi/Parashurama/Maa Gouri Sat– Shani Dev/Hanuman/Kurma/Shiva/Vishnu Rahu—Maa Durga/Varaha Ketu– Ganesh/Matasya Ascendent– Kalki
My question-Is this the only authentic & generally accepted method for determining one's Istha or more such methods are available in scriptures?