The eight kootas and their 36 points
The 36 gunas are the points of eight kootas, each worth one more than the last: Varna 1, Vashya 2, Tara 3, Yoni 4, Graha Maitri 5, Gana 6, Bhakoot 7 and Nadi 8. The heavier the koota, the more the tradition cares about it, which is why Bhakoot and Nadi together hold 15 of the 36 points and are the only two that can sink a match on their own.
Four of the eight read the birth star: Tara, Yoni, Gana and Nadi. The other four read the Moon sign: Varna, Vashya, Graha Maitri and Bhakoot. Nothing else in either chart is consulted.
Why the birth star and pada are enough
A horoscope written in India almost always names the janma nakshatra and its pada. The pada fixes the Moon sign too, including for the nine stars that straddle two signs, such as Krittika, whose first pada is in Aries and whose other three are in Taurus. That is everything Ashtakoota needs, so the star mode gives the same score the full calculation would.
There is one small exception. Vashya splits Sagittarius and Capricorn at 15 degrees, and two padas, the first of Purva Ashadha and the second of Shravana, straddle that line. When either appears the result says so, and the birth-details mode settles it.
How to read the total
Below 18 is conventionally not recommended. From 18 to 24.5 is acceptable, and most matches land there. From 25 is good, and 32.5 or more is very high, which the older texts treat with caution rather than celebration.
The total matters less than where it came from. A score of 22 with Bhakoot and Nadi intact is a sturdier result than a 24 that lost one of them, so the calculator shows every koota with the rule that produced it, and reports the Nadi and Bhakoot cancellations separately. A cancellation never adds points back: the total stays comparable with every other calculator.
What Guna Milan does not check
Ashtakoota never looks at the 7th house, its lord, Venus, Mars or the Navamsa of either person. Mangal dosha is checked separately, and a good score is not by itself a statement that either chart promises a happy marriage. For that, run the full Kundali Matching with birth details and read the two charts.