The calculator seems like a good estimation tool to go alongside that "200-foot rule" which is not a rule but a suggestion.
How does a person know what his antenna gain is? (i.e., 1.0 of whatever)
Even if one accurately knew the gain of the antenna system used with an unlicensed FM setup subject to FCC §15.239, it is highly unlikely that the operator of an unlicensed system would have the instrumentation needed for accurate measurement of the very low r-f power levels it would take for that antenna to produce the maximum legal free-space field.
Probably the best solution for those wanting to be compliant with FCC §15.239 is to use a transmitter/antenna system certified for this use by the FCC.
Having one or two certified FM transmitters opens several possibilities (in addition to trust in the certified units themselves).
Viewing the signal strength on a radio or spectrum analyzer one can compare the field strength achieved with a certified unit and field strength from transmitters like the Ramseys.
My two ceritifed FM transmitters, C.Crane and Wholehouse 2.0, look the same on a spectrum analyzer, giving added confidence that they are both adjusted to the same standard.
When I fire-up my Scosche, although also certified, I changed the antenna and it tends to be stronger than the other two, telling me that it is probably putting out too much power. So I have it side-tracked pending further tests.
When I'm experimenting with FM transmitters, I merely compare the range achieved by the one in question against my certified, factory tuned, Decade MS-100.
A good friend pointed out that this calculator contains a major error. It does not take into account Maxwell's Equation.