If someone can facilitate this passage from vettius valens, it would be great as I find it a little hard to understand just like many other old passages
I can explain the method, but I claim with certainty it is useless. Any rectification method to find the rising sign based on calculations is doomed to fail from the start.
The only way to rectify the rising sign is based on real life events and personality and maybe physiognomy. Rectification is a difficult business.
Now there are a few methods to find the rising degree in Valens, one is by Thrasyllus, another one involves conception chart, which is from where the tritutine method derives. I claim with certainty those are also circulatory, useless, illogical and impossible, especially those where you calculate the conception time with only a rising sign.
You must realise that if you calculate a conception time based on the rising sign, you
automatically claim a rising degree for all nativities that have the same rising sign at the time. Those who use those methods just deceive themselves.
If the conception time was not based on the chart with a rising sign, and was instead measured in real life, it is possible there might be some correlation.
Also there are twins that are conceived at different times. Not to mention that earlier than 7 month birth births are possible today, which is not accounted in the ancient sources.
Otherwise, rectification by calculation is impossible or just a simple guess, as I have demonstrated. A popular degree rectification method is to take a primary directions event and rectify it to the minute. You can quickly spot the astrologers who do this by claims of exact birth times to the milisecond, as if there was a stopwatch at the nativity.
Nick Kollerstrom's online article
"The Star Zodiac of Antiquity" 
has an interesting graph that compares the western sidereal value
(Aldebaran and Antares at 15 degrees of Taurus/Scorpio)
with the Spica at 30 degrees Virgo zodiac (today's Lahiri value).
The Spica zodiac gives the closest correlation with actual placements of planets in ancient horoscopes.
http://www.astrozero.co.uk/astroscience/documents/nick_kollerstrom_star_zodiac_of_antiquity.pdf
The graph shows mostly raman-like, a few lahiri, a few aldebaran and a few tropical. Can we really say what ayanamsha they used based on their calculations that are on average -+5 wrong? I have tested quite a few charts in Valens and different charts agree with different ayanamshas better. Very few, but there are some that work only with tropical for example. I would rather take their statements of fixed star placement, which is not prone to calculation errors (not really, but less likely, for example all tropical fixed star placements in Hellenistic astrology are wrong, because of erroneous rate of precession, but not due to wrong tables - i.e. proposed Stobart tables for Valens' charts). Manilius and Valens say the Pleaides rise at the 6th (remember ordinal) degree of Taurus. That is not possible in Lahiri or Raman. Also the latter put Regulus in the Venus bound of Leo, which is contrary to the delineation given in Valens of the Jupiter bound which extends from the 1 to the 6th degree of Leo.
Let's be careful in assigning 20th century Indian zodiacs to Hellenistic astrologers. There is one zodiac, however, that is ancient and agrees with the fixed star placement by Manilius and Valens, also it is the source of both System A and System B, of rising times, of tables and a large part of Hellenistic astronomy in general. This is the Babylonian zodiac.
(Note that taking Theon's formula for granted, produces Raman-like ayanamsha. However, I personally doubt that the same exact formula was used by all Hellenistic astrologers, although I have to admit there evidence on the use of later ayanamshas. At least they did not use Lahiri. Still, I would like an explanation for Aldebaran 15 fixed star longitudes.)
