In is the default. Out takes certainty. It's only a point.
If you aren't sure a ball is out, it's in. If you think it was out, it was in. In is the default, unless you're sure it's out. No discussion.
If you and your partner can't agree, it was in.
It's only a point. Call it the best you can, given that you were probably chasing the ball and it was moving fast.
Who has time to look down at the line and process where it landed when you're trying not to hit the fence, chasing a lob that sailed over your partner's head, after looking into the sun just long enough that the only thing you can still see is the ball?
If I hit the ball back over the net and then call it out, it's because my brain is old and slow, not because I want an advantage. If I can hit it, why would I call it out if it was in? I just want to play. But out is out.
Sure, I make mistakes, just like everyone else. We're playing for fun, for exercise, for the camaraderie, and because we love this ducking game.
The other side's lines are theirs to call. Don't play line judge from your kitchen line (29 feet from their baseline) or your own baseline (44 feet). Just move on to the next point.
No one will ever argue that a ball was in. Plenty of people will argue an out call they felt was in, even when nobody could see the line because the net, your partner, or a leaf was in the way.
The calls themselves, Out and Bounce included, are on Talk to Your Partner.