Most people cannot do this mentally, which is why a "Pranapada Lagna calculator" (manual formula or software) is essential. Here is the logic the calculator follows:
(Taurus, Leo, Scorpio, Aquarius): Add 240 degrees (8 signs) before adding the Sun’s longitude. pranapada lagna calculator work
If you are manually trying to verify how the calculator works, watch out for these three traps: Most people cannot do this mentally, which is