You seem to know a fair bit about manual resources, 'wrench.
Am I correct when I have a hunch that the AI overbuilds mines to a high degree? Ignoring fuel, my automated Constructors have about 17 sources for each of the core ship building ones. That is with some 50 colonies along a 12-3 o'clock sections of the ring in a ring galaxy.
I would think criteria for getting another mine for a resource is:
1: I want spare resources, not demand. Small demands on an otherwise well stocked resource probably won't matter and will resolve themselves.
2: Even if no demand, I want a spare. 2-3 sources never hurt no matter how little is actually used.
3: I want "common" resource sources scattered around my empire. Not every source in short travel around my capital. More areas have close supplies, and a war on a front won't suddenly wipe all my sources.
Typically, if I want a "district" away from the core worlds to be able to build a decent amount of ships, I probably want a medium+ space port in the area (I think they will hug more spare resources), and a source for each of the ship resources in that area. And of course, probably more than one source for the high demand ones.
Also it seems a good idea to try keep resources near each other. That makes them easier to defend. The good thing about the AI overbuilding them and spreading them around a bit is that it keeps pirates away, since they need an empty system with a fuel source...
I also have a hunch that you don't actually need many construction ships at all. At least if you don't have debris fields (I hate those, so never enable the original storyline). In my current (stale) game, It think I had a single AI run constructor until 20+ colonies.
Also. When there is a mix of stores and demand. Is there a simple way to figure out where the demand is? Scanning the cargo of all colonies and bases is not simple...