Note this rule is taken by book, Advance Predictive Technology Of Astakvarga written by M.S. Mehta and R.C. Dadwal.
1)Add SAV of 10th house and SAV of 6th house.
2) Divide by birth Nakhshatra.
3)Note the reminder. And Add reminder with SAV of 10th house.
3)Now subtract this with 27. Note then reminder. And count from birth Nakhshatra and note the Nakhshatra.
4) When Sun transit over that nakshatra or 10th house lord transit over that nakshatra then promotion in service will be possible.