The way it works now seems reasonable, when you first see the monster (at 50-60 tiles), it probably doesn’t see you, so you’ll probably just ignore it and go back to what you were doing. Once it’s closed by 1/2 that distance (25-30 tiles), it’s probably closing because it sees you and is intentionally approaching, we want to warn before it’s right on top of you so you have a chance to respond. You might or might not get a third warning, but if you do it’s right on top of you (12-15 tiles). Let me put it this way, if it’s a moose, do you want to go straight from ignoring it at 60 squares away to having it warn you at 15 squares away (or worse, go from 40 to 10)? That seems a bit close for comfort.
If anything, it might be reasonable to expand the exclusion zone from 6 tiles out to 10 tiles or so to make sure you don’t get one more pointless interruption. I’m also thinking we want to scale this with view distance, if you spot something lurking at the edge of your flashlight range and ignore it, I think you won’t get even a 2nd warning.