Question: how long before fajr is tahajjud?
Answer: - the last third = 1:45 am to 4:25 am, - the fourth sixth = 12:25 am to 1:45 am, - the fifth sixth = 1:45 am to 3:05 am (80 minutes before Fajr adhan). Based on the previous discussion, you can pray Tahajjud anytime during the night on condition you pray it after waking up from sleep and before the adhan of Fajr.