This is only for Ishaa'. For fajr the time will be 18 deg. A one year actual sighting was conducted in Montreal, Canada which concluded that degree of elevation was indeed 18 deg. In Fajr, it is a specific time for the dawn, hence no ikhtilaf. You will finish your sehri according to that time. So basically, if in time of difficulty the change will be in Ishaa' degree and NOT in the fajr time calculation.