Edit: I made a better one here: https://factorioprints.com/view/-MPRrQy0RB9QOWXrKUq7
Original: Side load/half belt input: it doesn't matter which side items are loaded, it will be automatically adjusted. Just make sure things are going in. It may take some time to become stable, but eventually everything will be working properly.
504spm in theory
Factorio Forum @Eul_S