There are two different forms of setting though aren't there. The archaeological setting and the site's (possible) contemporary setting. The difference is only minor and hard to quantify, but the latter needs a fixed point in time. The former requires a timeframe.

>> And of course, once you start on about ruining the view it hands them the chance to say No problem, we'll reinstate everything.

Obviously the setting must include all archaeology within the viewshed :-)

I suppose a further addition of something like : "As far as a continuous band of relative archaeology continues"

... allowing a reasonable, but not fixed, distance between sites. Then, for example, the setting could be a circular area with a couple of spidering sections to take in roads or settlements.

Defining the setting of a site must be done on a site-by-site basis, but you have to start with a minimum somewhere.