The rulings connected with prayer times (e.g. fajr starting time) are determined by seeing with the naked eye.
It is not correct that one should overburden oneself by meticulously following timetables based upon astronomical calculations.
Rather, the Sunnah is ease and this Ramadhan timetable is merely an approximate guide.
See ‘Majmoo’ al-Fatawa’ of ShaykhulIslaam Ibn Taymeeyyah (25/126-202), ‘Al-Majmoo’’ of an-Nawawee (6/279), ‘At-Talkheesul-Habeer’ of Ibn Hajr (2/187-188).