This computation follows the Jaimini method: the planet with the highest degrees in the chart is the Atmakaraka (the soul's significator); the sign the Atmakaraka occupies in the Navamsa is the Karakamsa; and the planet in (or ruling) the 12th house from the Karakamsa — the gate of liberation — indicates the ishta devata.
The planet-deity link follows the BPHS Dashavatara tradition — Sun→Rama, Moon→Krishna, Mars→Narasimha, Mercury→Buddha, Jupiter→Vamana, Venus→Parashurama, Saturn→Kurma, Rahu→Varaha, Ketu→Matsya. It is a classical pointer for one's sadhana; the family deity's place always remains intact.
Are the ishta devata and kula devata the same?
No — the kula devata comes from family lineage, the ishta devata from the chart; worship of both runs together without conflict.
Why the 12th house specifically?
In the Jaimini tradition the 12th from Karakamsa is the moksha-sthana — the deity of the very gate through which the soul is freed is held most fruitful for sadhana.
What if no planet sits there?
Then the house lord gives the indication, and if needed its Navamsa dispositor — the calculator applies this order automatically.
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