I ride all oxers to the front rail. The distance inside the line is measured from the back rail of the first jump to the Front rail of the next one. Other than keeping my horse round and balanced up, I don't consider the back rail at all. Well, unless I am in the jumper ring and it's a behemoth spread then I make sure I am round,balanced AND have enough power to create a large enough arc. In the hunter ring oxers aren't wide so the arc of jumping the fence properly will take you over the back rail. Don't look down and don't lean out.
I can explain it TO you,but I can't understand it FOR you
I look at the front rail too. I ride jumpers. I press out over the wide ones, but other than knowing that I need to ride over/past the jump, I don't really think about the back rail.
ETA I just saw your other thread and I see you're looking for insight into how people ride oxers. Don't lean, don't look down. Do concentrate on riding over and past your jump, focus up ahead where you want to eventually be. If you have a good quality canter and are nice and straight, looking ahead and up, you will be fine.
I've been out of the game a long time, but doesn't it depend on the horse? I always rode my junior jumper to the back rail, but she also had a very quick and careful front end, but not excessive scope. The rule with her was always a fast pace and distance choices of deep or deeper.
I would have ridden a scopey horse that needed space in front of the jump very differently.
I ride to the front rail, as that's where the distance is measured to, but I also like to ride my oxers slightly deep so that he is at his peak at the back rail. If I rode an oxer like a verticle, he'd take off too soon and he'd drop the back rail.