Today’s “Fateful Showdown” event wasn’t the first time where the devs set a wrong number as the boss hp. We’ve had Avacyn event ending in just hours or lingering for 3 days.
It is obvious that it’s a hard task to predict how active the playerbase will be at any given event, that’s why the boss health shouldn’t simply be a number set in stone before the event starts.
Instead I’d suggest that the event boss hp number would be generated or adjusted by the game at some point AFTER the event started, based on the active players and/or total damage dealt in the first hours.
I believe the optimal point at which the final amount would be determined would be the 2nd recharge of nodes - 16 hours after the event start (while at 8 hour node recharges). This would get the most accurate information (all players caring about not burning any nodes would have played at least a bit till then, no matter their timezone).
For those first hours the game should use an estimate boss hp (just like now) except there should be a safeguard cap preventing a loss of more than a certain % of health during that period.
Then at the 16th hour (second node recharge), based on the total actual “damage dealt” (ignoring the cap), or based on the number of players who played at least 1 game on each node, an algorithm should calculate the boss hp needed for the event to last long enough for the active players to have a reasonable chance at getting the top progression reward.
I know there are more factors to consider, would be too long to write about them all, but that should show the general idea.