Is it permissible to estimate the end time of Ishā' by the length of the night in hours, such as by saying the night is 11 hours long and the time for Ishā' ends after four hours of it?

The Shari'ah has established clear signs for the prayer times. The duration of the night begins at sunset and ends with the true dawn. If the sun sets at 6:00 and the dawn breaks at 4:00, then the night is ten hours long. Its first half ends at 11:00, and its first third ends around 9:29.