frak, that what I feared. My problem is given that the 'load/unload to set level' is toroughly bugged and generate lot of errors (almost unusable) I can't automate much some things. I would like to maintain a certain level of minerals on some colony worlds, but this is problematic.
I indeed found that you can have a freighter load everything from a colony and have a second one coming just behind and dropping a fixed amount, but this is not optimal, if there is a production cycle between the two, you get a shortage message.
Also, the problem is you can't partially unload minerals, you have to unload everything.