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.
Mars-- Hanuman/Subramanya/Sri Narasimha
Jup-- Vishnu/Sri Vamana/Dattatreya/Sai Baba
Ven-- Maha Lakshmi/Parashurama/Maa Gouri
Sat-- Shani Dev/Hanuman/Kurma/Shiva/Vishnu
Lets help ourselves.