According to the Salaat timetable here in Nashville, U.S.A., Isha begins 30 minutes after Maghrib.

Answer

Isha' is generally one hour 15 minutes after sunset. After sunset there remains a reddish glow in the western horizon. When this red glow disappears, a whitish glow appears. After disappearance of the white glow it becomes intensely dark. With the disappearance of the white glow, Isha time commences. This time differs from region to region. We do not know whether this phenomenon happens 30 minutes after sunset at your end. When it is intensely dark, then you may perform Isha'. 30 minutes do not appear correct to us.