When is the Last Third of the Night?

I would like to know how does one work out the "last third part of the night" for praying Tahajjud Salaah? Do you work out the difference between Isha to Fajr and then find the third portion or is it from Sunset to Sunrise?

In the Name of Allah, Most Gracious, Most Merciful,

The Arabic Lexicon, al-Misbah al-Munir, defines night as the time from sunset until the entry of the dawn prayer (fajr). So in order to work out the last third of the night, one calculates the time between the sunset prayer (maghrib) and the dawn prayer (fajr), then deducts two thirds from that time.

For example, if maghrib was at seven o'clock and fajr at four o'clock, one calculates the time between the two prayers, which is nine hours. Then, one deducts two thirds from the nine hours, which is six hours. Therefore, the last third of the night would begin three hours prior to fajr, at one o'clock.

And Allah knows best

Fadi Qutub