Please provide guidance on how Isha should be calculated at high latitude. There are problems with perpetual twilight and problems when Isha occurs but happens extremely late, can Harj be taken into account during these late Isha times and...
We are assuming that you are using accurate figures from a reputable site such as HMNAO. We do recommend that you add 5 minutes to the sunset times which are obtained as there are genuine reasons for it as explained below.

Fajar at 18 Degrees

The last major agreement of Scholars in United Kingdom on this subject was in 1983. This was conducted on the 6th Sha’ban 1403AH corresponding to the 29th of May 1983 on Sunday at 10 AM at Bradford. This meeting was attended by approximately 70 Scholars and more than 100 responsible members of various Mosques. The conclusion reached was that Faj’r Salah should be calculated at 18 degrees during normal days.