As far over the back wheel as possible, that keeps the front end light which makes it easy to initiate the slide, because the aim is for you to be pivoting on the back wheel with next to no weight on the front so its easy to bring it round. My backwards riding is not that great yet either, but i'm starting to get sliders pretty dialled, just takes practice, and then more practice until you dial it in.
As far over the back wheel as possible, that keeps the front end light which makes it easy to initiate the slide, because the aim is for you to be pivoting on the back wheel with next to no weight on the front so its easy to bring it round. My backwards riding is not that great yet either, but i'm starting to get sliders pretty dialled, just takes practice, and then more practice until you dial it in.