A short and simple guide to designing your bus the right way, the first time.
Guide to Main Bus Design Guide
Distance Between Belts
You want your belts on every 4th line. This is enough room for running belts between splitters, which may be necessary under cramped circumstances.
You want to use the odd numbered heights for your bus belts. This is because the 4-way splitters only connect at those heights. Use the even numbered heights for inputting/outputting resources using the other types of splitters!
Make sure to run the belt along the equator! Not north-south! Because of the grid lines, and also because you want the maximum distance for the bus to travel.
You'll know you're on the equator because it'll be a thick green line.
Don't overlook the different splitter types, like I did initially! Press Tab while placing to change to a couple different types. One of them in particular is very useful for inputting and outputting resources from the line, as demonstrated in the screenshots below.
As explained before, you want your bus belts running on the 1st and 3rd levels. So here's an example of how to merge from the 3rd to the 1st level. I did this in the smallest amount of space possible.
Alright, here you can see where I'm going with my bus. I started the first line directly on the equator. Every additional line will be added above that one, and all my factories will be below the equator.