Spanning tree itself doesn't have anything like what you are asking. As the network designer, you assign the priorities to ensure that the correct bridge becomes the root bridge. How you select which bridge should be the root depends on how you want the traffic to flow, and that is up to you as the network designer.
If the majority of your traffic is kept within the LAN, then, yes, you should pick the bridge closest to the center of the LAN. If most of your traffic exits the LAN, then you should pick the bridge where your router is connected.
You design the LAN topology to fit how your LAN will be used.
Per your comments:
The LAN diagram to which you linked is something that cannot be reasonably supported. Yes, networks sometimes grow in an unreasonable manner, but that network would need to be redesigned to introduce layer-3. We no longer live in the layer-2 world that existed when STP was developed, where it was, "switch where you can, route where you must." We live in a layer-3 world, and almost nothing requires you to have a large layer-2 LAN.
You can drive just about any protocol to its limits, but that should be avoided. Recent best practices really limit the usefulness of STP to be a failsafe because depending on STP can make your network more fragile, and experiencing an STP problem will render a LAN useless. With a LAN, such as you depict in the drawing, a business can lose millions of dollars per hour/minute/second due to STP problems, which are notoriously hard to correct. No sane business will allow such a LAN.
STP, itself, has some default values for things like diameter, and you change those at your own peril.
You question claims, a few times, that the root bridge is selected by bridge ID and MAC address. It is selected only by the bridge ID, of which the MAC address is part. The bridge ID is the bridge priority plus the MAC address, and the MAC address is not considered separately when selecting the root bridge. The most significant part of the bridge ID is the priority, and you need to configure the priority if you want to determine (and you should want to determine) which bridge becomes the root bridge. The MAC address is only significant if there are identical priorities.