All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
The determination of the beginnings of each Hizb and Juz’ of the Quran is an issue of Ijtihaad (scholarly reasoning) and not an issue that is Tawqeefi (i.e. determined only by the Quran or the Sunnah and not amenable to personal opinion); it is for this reason that you will find that it differs from one edition of the Quran to another and from one publishing establishment to another.
Az-Zurqaani said in Manaahil Al-’Irfaan:
“The 'Uthmaani copy of the Quran was not divided into parts, and it was also devoid of dots and diacritical marks. As time went by, people started to divide the Quran into parts in different manners. Some of them divided it into thirty parts and named each part a Juz’, in a way that when this term is mentioned, you would not think of any other thing than the Juz’ of the Quran. So if someone says I read a Juz’ of the Quran, one will understand that he has read one of the thirty parts into which they divided the Quran.
The people who divided the Quran into Rab’ahs followed this division (into thirty parts); they printed each Juz’ as an independent copy and they call the totality of the copies a Rab’ah. These independent Juz’s (parts) are made available to young students in schools and others.
Some people divided the Juz’ into two Hizbs. Others divided the Hizb into four parts and they named each part a Rub’ (quarter). Others write the word Khams (five) at the end of every five verses of the Soorah (chapter) and the word 'Ashr (ten) at the end of every ten verses of the Soorah. After five other verses, they put again the word Khams, and when they reach 10 verses, they put again the word 'Ashr, and so forth until the end of the Soorah.
Some of them write the letter خ at the end of 5 verses instead of the word Khams, and they write at the end of 10 verses the letter ع instead of the word 'Ashr.
Some other people write the number of each verse from the Soorah at the end of the verse and some do not write a number. Others write an introduction to the Soorah mentioning the number of its verses that were revealed in Makkah and the number of its verses that were revealed in Madeeanah, and so forth.”
Allaah knows best.