Hmm, how do you account for that actually? Is it more like "this is where my crosshair should be" kind of thing?
It's just the idea that your crosshair should always be in a position to shoot without too much correction, yeah. The most common problem is that people keep their crosshair too low while moving, so that they have to move it up when somebody walks into view. I know Overwatch isn't like CSGO or something and there are varying heights and hitboxes and blah blah blah, but it's generally easier to correct downwards anyways.
So like with McCree I keep the crosshair around head height always, if Torb or D.Va or something comes into view it's easier for me to aim down for correction anyways. With Hanzo you keep it slightly above head height for the range you think you're gonna be shooting at. So on and so forth.
You also wanna know where it's most likely somebody will be standing when you peek a corner or whatever. Like I know on Numbani the different spots where people stand high ground, so as I turn a corner I just have the crosshair already at head height relative to that high ground.
Its also important to have versus somebody like Genji. If Genji jumps you, it's not a bad idea to just put your crosshair around where his jump will peak and just correct left/right. Obviously it's not that easy and he can not jump or whatever, but that's part of what you do before you just go for the flick.
It all sounds super obvious and very easy to do, so a lot of people stop actively thinking about it and fuck it up. If you just remind yourself whenever there's down time and you're about to do something it gets ingrained and you can think about it a lot less.