TL;DRChoose the direction of the Underground Belt (Up or Down) based on what is in front of it.
What ?Instead of defaulting an unconnected Underground belt to be a Down belt base it on what the tile it will be interacting with is. If its a belt going away or sideways from the underground make it default to an Underground Up. Here are some examples:
Why ?If the belt is away or alongside the nderground it probably means you want to unload from the Underground or side load from it, so default the Underground to the direction that is more then likely intended.
I'm making this as I find I build most of my underground from the destination to the source of the items, so I end up having to manually reverse the direction of all of them.