When he first used his quirk on Deku and explained how short the timespan was, I immediately wondered why he was able to predict AM's death.
Then I came up with the idea that it might be the more he spends time with someone, the longer the timespan he gets.
It might also be a function on the number of times he uses his quirk on the same person.
My guess is he used it a lot on AM for logistical reasons: By knowing where and when AM fights villains, it's easier to get the police, ambulances and support at the right places at the right time.
But then at some point he's seen AM's death...

And I also understand why he doesn't want to check everyone's future, because he knows some of the heroes present at that meeting will die. He doesn't want to experience it from a first person POV and he doesn't want to feel powerless as he probably never has been able to save someone he saw dead in his visions...