I usually call people by the names they wish to be called; simple as that.
If they’ve a nickname they like, I’ll use it. If it’s a nickname they don’t actually like that much, but tolerate other people using, I’ll use their ‘real’ name.
In ‘real life’, almost no-one knows me by my ‘real’ name. My writing may be credited to my real name for legal purposes, but almost no one I write for, none of my friends, none of my friends’ kids, my lad’s girlfriend, pretty much no-one knows me by that name; they all know me by a nickname I picked up at university 35 years ago. (In part, I grabbed onto it because I always disliked my real first name ; it’s kind of androgynous. No, not Leslie, but it serves as an example. I’ve known both men and women called Leslie, with that spelling. My real first name is similar.)