In places of high latitude that experience persistent twilight, how do prayer calculations (like Islamic Finder for example) calculate when to apply the special calculation methods (whether it be the angle, nearest time, nearest latitude, 7th of night, etc. method)? It doesn't appear that the calculations wait until the natural time of Isha disappears but rather, as the time for Isha gets smaller, the alternative calculation method comes in to make a smooth transition.
Looking at the data from the Islamic Finder site, it appears that they're doing some sort of angle calculation and that when the duration of isha under the angle calculation exceeds the length of the natural isha duration, the angle method is chosen. But I really don't know for certain.