Assalamu alaikum ustadh. I have heard that Umm ul Qura calculation is wrong for Isha it adds a fixed 90mins after sunset. When I calculated, it does come to 90mins But I think all the masjids here (Saudi)follow this. What should I do? If I catch the prayer according to this, have I missed

So the people in Saudi are praying wrong? I don't understand. You have Sheikh Al-Fawzan, Sheikh Al-Abbad, the senior scholars, the Council of Senior Scholars, and all of these people, and they didn't know, but you knew? Or maybe this is OCD, or I misunderstood and there's something else you meant?