Importance of 64th Navamsha:
The 64th Navamsha from Moon holds special significance. This is because, when malefic planet transits the 64th navamsha from moon, it gives sorrow to the native.
Method to calculate 64th navamsha in horoscope:
Take Navamsha kundali. See where the moon is in the namansha. count 4 houses from the moon. This will give you the 64th navamsha for that kundali.
For Example: If the moon is in Aries in Navamsha, calculate 4 houses from there. 4 houses from Aries will be Cancer. So the 64th navamsha will be in Cancer. While analyzing transits, the transit for malefic planets over cancer should be analyzed for identifying problematic timeframes for this particular kundali.