It is true that the minute division of a Rashi by 150 brings, difficulty, to know Nadi Amsa for a birth chart ascendant.
But when a Nadi Amsa chart is made with unequal Nadis, then the new Nadi ascendant and the position of planets in different signs, makes it easy to nail down a chart for a person by reverse-analysis, as done in Nadi Tamil Palm leaf.
A Nadi Amsa chart is unique to a birth.
And with the Nadi calculation of timing based on a 120 years cycle, which is NOT Vimshottari Dasha, makes reading on a yearly progression basis. Simple.
Example : My , Chandra as 7th house lord for marriage is in Mesha in Nadi Amsa (D-150).
At 30–31 years Karkataka (Cancer) Nadi and sub-nadi Thula was running.
Thula seen by Chandra from Mesha sign as above, made marriage to happen, by fate.
In this type of reading, there is No need of applying dasha-antara lord logic to arrive or predict an event, like present day astrologers do, which is shoddy.