I can't find an explanation of exactly what the numbers mean. just use examples from vanilla RFC or DoC but not SoI, those numbers are different.
after having done a couple of maps I think it easier to just label the major city sites in the WBS and leave the connecting tiles. I think anything under 90 doesn't matter.
Just seen this one now, but thought it might be useful to have Rhye's explanation of the numbers from the vanilla RFC maps:
3 - FORBIDDEN: this civ should never found a city here
20 - NO DATA: this region has nothing to do with the civ, so it's bad to found here
40 - territory in which settling wouldn't be bad, but the presence of another civ is more important here
60 - possible unhistorical expansion
90 - territory in which settling would be good, but there is some problem (it would steal plot to another civ)
150 - limit territories of historical expansion (sometimes without city name)
200 - limit territories of historical expansion (sometimes without city name)
300 - territory of historical expansion
400 - surrounding of core territory
500 - core territory
700 - very important plots that should always be founded if possible
In general I think the AI will settle under 90, but only if there are only these plots left, and only if the AI can find no other priorities. If it sees a tile with 400+ then it will immediately build a settler to found it, for 100-400 it will build a settler once its cities have basic infrastructure, but less than 100 it would never found unless it had built all possible improvements, had an army stronger than any rivals, and had no real option to build other than to expand.
So in practice most cities under 90 never get founded, but you can only be certain a civ will never found a city if you put 3. In fact, Rhye's vanilla map for Rome (for example) features numbers 40, 60 and 90 in North America, so we could assume that means that the AI would potentially have settled those after discovering Astronomy, although AI Rome would be unlikely to live that long and have the opportunity.