السلام عليكم, With regards to Fajr time, as far as I know, there isn't a scholarly difference of opinion of 15 vs. 18 degrees. The scholars are agreed on the observable phenomenon in the sky that marks the beginning of Fajr time. The question is just how to calculate the time of that phenomenon accurately- that is, it is a matter of astronomy rather than jurisprudence. The way to be certain is for someone, who knows how to tell the start time of Fajr from the sky, to go out of the city in your locality and mark down the time when they observe Subh Sadiq and compare that to the timetable. even if your local masjid distributes 15º timetables, it is necessary for you to stop your fast at 18º. It's not a matter of scholarly opinion, a person can always go out with someone who knows how to recognize Subh Saadiq and observe the sky for themselves.