I was try to get some reference from any classical texts to find how much karma the person has gathered thru free will in this birth versus the destined karma results from previous births.
Prashna Marga Chapter One gives this details. It mentions that Natal horoscope at birth indicates all the karma done prior to birth. The prashna chart cast at the time of prashna can be used to find the additional karma the person has accumulated since birth till that moment. I think the older the person gets more the chances of accumulating additional karma. A young child may not have accumulate much karma by free will whereas an older person could have underwent most the karma fruits indicated in the birth chart and also may have added additional karma. So as the person get older I think it's very essential to cast the Prashna Chart. I am giving the qoutes from Prashna marga Chapter one below.
Stanza 39. — The balance of good or bad Karma brought forward from the previous birth is prarabdha, and it is the reading of this that goes under the name of Jataka or Astrology.
Stanza 40. — One undergoes the consequences of one's previous Karma from birth to death and this can be known from his horoscope. Then what is the use of Prasna?
As the name of the book implies, Prasna Marga has primarily to do with Prasna or horary astrology, though, almost all the principles may be used with advantage in interpreting radical horoscopes. In stanza 40, the author queries as to what use a Prasna chart could be put to when horoscopy can deal with all important events. He answers the question in the following stanzas: -
Stanza 41. — Prasna reveals to us whether a man reaps the fruits of his deeds in his previous birth or the fruits of his actions in this birth. Here a question arises.
Stanza 42. — How can we distinguish whether it is the result of our previous Karma or present Karma that is yielding its result now?
Stanza 43. — If the horoscope has bene6cial planetary patterns and the Prasna chart malefic ones, then it is to be understood that the native is experiencing the fruits of evil Karma done in this birth. If vice versa, then also the native is experiencing the effects of good Karma done in this birth.
Stanza 44. — If the planetary patterns in the horoscope as well as in the Prasna chart are similar, then a clever astrologer should divine that the native is experiencing the fruits of his past Karma only.
In stanzas 41 to 44, the author explains the relative utilities of the Prasna and the birth charts. There is repeated emphasis on the influences of Karma and the importance of Prasna in finding out the nature of Karma to be enjoyed in this birth. The horoscope by itself reveals the nature of past Karma and the Prasna chart acts as some sort of a supplement to the birth chart. Suppose we see in a man's horoscope a good period and in a query of his (Prasna chart) a very bad time, then we have to assume that the person is reaping the bad effects of his Karma done in this life. Suppose we see in his horoscope a bad period and in his Prasna chart a good period, it is to be inferred that the person is reaping the favourable effects of good deeds done in this birth only. If the horoscope and Prasna are similar in positions and combinations, then the person is reaping the result of Karma done in his previous birth. Since some sort of a balancing of the birth and Prasna charts is involved, it is clear that the current indications in the birth chart, i.e., the indications obtaining at the time of a query — the benefic and malefic dispositions of current Dasa lords, Yogakarakas, etc., have to be studied along with the Prasna chart to know the nature of Karma now being experienced.
SIMILARITY BETWEEN PRASNA AND JATAKA
Stanza 45. — The Prasna chart should be read just as the horoscope is examined with reference to its relative merits and demerits.
Stanza 46. — A person goes to an astrologer prompted by Providence to know his future.
Therefore there is a close similarity between Prasna and Jataka.
Stanza 47. — As Prasna Lagna is similar to Janma Lagna, all events should be read from Prasna as you would do in a horoscope.
The sum and substance of these three stanzas is that the time of question (Prasna) should be given the same importance as the time of birth. Human births are regulated according to the law of Karma and hence the time of birth is significant. Similarly a person proceeds to an astrologer to ascertain his future prompted by a Divine force and hence the time of question is equally important. There are no doubt certain points of difference in reading Prasna charts and horoscopes. They are discussed by the author in their proper places. But unless otherwise implied, all events revealed in a horoscope can also be read from a Prasna chart. The principles given in this book may be used with advantage in studying horoscopes.