This is the parts yard I used for making the Hendekabelt mall. The main items of interest are the multiplexers. They're all labeled with belt icons to show the items/minute that will come out of the output side once everthing is hooked up. Do NOT upgrade or downgrade the belts for those when you place them, or the proportions will change. (Unless, of course, you're experimenting!) Try hooking up some of the plexers in sequence for more finely-grained proportioning!
There's also an assortment of vertical belts and stair-step belts to make routing your belts around and over and under various obstacles a lot easier.