How many times to create the same item?

I ask myself how it works best - do I make one factory for e.g. Rotors and then ship them to my 'Motor' factory and also to my 'Smart Plating' factory or do I just create them twice on site?

Same for everything lower tier I assume. I think I have to make a cut at a certain level - not shipping iron rods around, but not so sure with rotors.

How do you guys usually do it?