Hi Gladman,
The rule of thumb should probably be to create a single TMA site for monuments of the same type within a short distance (less than 1km) of each other. You can add more than one site if there are different types of monument (e.g. standing stones and cairns).
We're trying to reduce the amount of new multiple sites created for individual monuments that are really all part of a single group, so a group of cairns should be added as one TMA site if they're close together.
We're also slowly reducing the number of sub-sites on TMA, as this causes confusion and leads to duplicate sites being added where people don't know they already exist as part of a sub-site - the group you found in Dartmoor recently is a good example of this. Please could you add your site(s) directly to the county?
Hope that helps and look forward to seeing your posts.
TMA Ed.