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Thanks for the very good and clear writeup of the part 15 FM situation.
I do not intend to detract from what you posted, though physical reality might.
The weakness for measuring field strength is in the receiver and receiving antenna since the actual sensitivity of the receiver and the gain of the antenna are not known (to at least this) consumer. I have no idea of these specs. for my radios.
Even if known from factory specs., a slight deviation due to manufacturing tolerances may vastly affect the range and therefore the assumed field strength. Consider that a receiver specified at 3 uV sensitivity may actually have a sensitivity of 1.5 uV for a certain degree of quieting. This would translate to half the allowed field strength allowed by the rules. Perhaps the antenna or coax diminishes the signal delivered to the receiver by 1/2. This, unless known and accounted for, would lead one to believe that the 3m FS is OK when in fact it is double the limit.
It is a difficult problem for folks with average equipment and information to assess compliance. True, the 200 foot suggestion is not much in the way of guidance, but it is probably in error on the conservative side.
Your ideas are valid, but I fear the unknowns at the receiver end make reasonable estimates of the field strength difficult.