Is what the questioner did—investigating prayer times in Finland—religiously permissible, and can he pray at the earliest Fajr time he adopted for abstaining [from food/drink], or must he wait for the later time which he excluded?

Prayer times are defined by religious texts and depend on observation: Fajr at the appearance of the true dawn, Dhuhr at the sun's zenith, Asr when the shadow of everything becomes equal to its height after the shadow at zenith, Maghrib at the complete disappearance of the sun's disk, and Isha at the disappearance of the red twilight. Whoever cannot determine the times should follow those who know them or rely on calendars. In case of differing calendars, the more cautious approach should be taken.