Dear Vijay and others,
Rectification using Kunda was taught with examples in the first few chapters of Pt Sanjay Rath's excellent book "Crux of Vedic Astrology: Timing of Events". This is such an excellent book that even the most learned scholars will find something or the other in the book that they don't know.
You can also find the longitude of "Kunda" given in JHora software.
WARNING: Read below only if you are interested in my honest personal view on Kunda. I am not looking for an argument on this contentious issue. I am mentioning my views only to benefit those who care for my views.
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I personally don't think much of Kunda method of rectification. If you think logically, you too will realize that it cannot be correct. Definitely, there is something amiss with our current understanding of it.
Most rectification methods divide time into various quanta. Human birth can occur in some quanta and cannot occur in some quanta. But these quanta are so large in some methods that it is just absurd. Kunda is one such method.
Lagna changes sign once in every 120 min on average. So Kunda changes sign once in every 120/81=1.48 min.
This results in a 1.48 min window when birth can occur and a 4.44 min window when birth cannot occur. This will be followed by a 1.48 min window when birth can occur and then a 4.44 min window when birth cannot occur.
We have instances when twins were born 3 minutes apart. We even have instances when triplets or quadruplets or pentuplets or sextuplets were born onc every 2-3 minutes.
If you are told that twins were born at 1:03 pm and 1:05 pm, you may have to change those times to 1:03, 1:08 or 1:02, 1:03. The way I look at it, it is just absurd.
The quanta of time used in any genuine rectification methods must be much smaller than 1.48 and 4.44 min. The tattva and antara tattva method seems more reasonable to me.
May Jupiter's light shine on us,