When constructing our factory we quickly ran into a very frustrating problem: We could only split lines 50/50.
I'd love to see a very precise splitter where I can set a ratio on the exits. I want to go up to a splitter and press 'E' to configure it. A little GUI appears where I can see all three exits. There I can see all the items that are currently running into this splitter listed next to each exit. Next to the item names is a little text field where I can type in numbers to define a ratio for each respective item.
Let's say I need 40 tubes to cover my need for screws and want to split off the rest of my production into some storage box in case I need the screws for something else. I can see in my Constructor interface that I am producing 56 per Minute. I could enter 40 on the exit that leads to my other Constructor and 16 on the exit that leads to my storage box. Or I enter 5 and 7 because I can do math. The splitter then calculates its ratio and distributes the items according to this ratio.
Simply "overloading" the system with parts, as not required materials are just stacked back, does work of course, but leads to a way bigger facility for no real reason.
This is one aspect that makes this game, that I enjoy fully otherwise, a little bit frustrating, as proper item flow management is not really possible without this.