Assalamu Alaykum Ustadh, may Allah grant you goodness. Most of the Salafi Mosques use different calculations for Fajr and Maghrib than all the other masajid. Which masjid should I follow for those prayer times, the open Salafi Masjid or the one I go to the most (because of work - distance).

You have to follow the one that prays at the correct time. If the one closest to you is the one that prays at the correct time, you can go to it. Even if they have different timings, that is not a problem as long as it is within the correct time. But if it is not, and the other one is far away, you can pray at home in congregation. It is not obligatory upon you to go a long distance (like a 15-minute walk or 10 minutes by car) to pray in the masjid. You can pray in congregation at home.