While there are 12 zodiac signs, there are 27/28 nakshatras. Therefore, calculations based on nakshatras are more pin pointed*. Stellar Astrology uses mathematics based on nakshatras. To get a flavour of calculations done in Stellar Astrology, the sample calculations .pdf file can be found at this link.

*360° divided by 27 gives finer value in comparison to 360° divided by 12.