I’m not sure if I understand your first sentence properly. Are you saying that sometimes a low tier stone tree ring like MP has is used as an alternate to cutting a tree down during some situations of property redevelopment? If so, I’ve never heard of that and curious to see a real example of where that has been used.
In times of property development, I only know of tree protection zones employed to protect sensitive tree areas during the construction periods. It’s square fencing and gets removed post construction. They protect the tree from equipment hitting them or causing issues very close to the flare of the trunk, but don’t do much to protect the tree from wider root injuries resulting from digging or compaction.
For the diameter of the tree mulching extending to the diameter of the crown (i.e. drip line), that works easily for slender trees in residential lots, but some of the wider branching trees can have a crown diameter of 50-100ft. If I’m creating a tree ring for a mature tree on a residential lot, I’d typically go with what looks aesthetically balanced with the tree in relation to the property around it. For a front yard like MP’s that might mean a 8-10ft diameter tree ring for each one. For an estate property that has the space, go wide and make that ring look as grand as the tree.