Ishta Devta is one who helps in getting salvation or moksha. Navmansa plays important role in finding out Ishta Devta in a horoscope as it is also called dharmamnsa. Planet having the highest longitude is known as Atmakarak. The position of atmakarka in navmansa chart is karkamnsa and ruling deity of 12th from karkamnsa is Ishta devta.
Some astrologers give importance to fifth house of a horoscope to find out Ishta Devta. In case longitudes are not available, deity governing the lord of fifth house or devta of the the planet posited in fifth house whichever is stronger should be considered as Ishta Devta.