Magic, Dragons & Time Travel (oh my)

I struggle with this sometimes. Where do you draw the line between “it’s SFF, so anything can happen” and “this doesn’t feel believable?"

I’m a science/tech nerd, so my brain keeps trying to build logical scaffolding for everything, which is hilarious considering I write magic and time travel is my favorite trope to read.

How do you handle this in your own work? What makes you accept the impossible, and what line can't be crossed for you?