Astronomy vs. Astrology

Astrologers say Lagna is more reliable than traditional 'zodiac horoscopes'.
Lagna is taken from start of the day. The lagna at sunrise is the constellation where the sun is supposed to be that time. Two hours later the constellation rising next to the first lagna of the day is taken as the next lagna. The lagna changes every two hours just like this. But if we are taking the reference point at sunrise and the constellation the sun resides in at the sunrise, then we are purely not using the right constellation. It is because no astrologers are updated with the precession of equinoxes. The calculation is again off by one constellation. So lagnas calculated for any time of the day is wrong. So how can the astrologers be so sure that when they are calculating lagnas they are making more accurate predictions?