The UMR EMC lab javascript adds an entry for mr. This calculator assumes a value of 1, which is a very good approximation for just about any wire you will be using. You can use the original at the link provided, as long as they don't move it or take it down. This calculator matches their results for mr = 1 in both the rectangular and square cases.
Parasitics in the loop - inter turn capacitance in particular - are not taken into account. They will limit the high frequency response of your loop, so the estimate of maximum frequency is probably off.
About the wire radius - this calculator seems to give more realistic results when the "wire radius" is actually half the spacing to the next wire - in other words they seem to have assumed tightly packed magnet wire. The default values given are for my "ribbon cable" loop, which has 26 AWG conductors spaced 0.050 apart. Obviously I did not enter the radius of 26 AWG wire, I entered half the winding spacing instead! I do not know how well this calculator holds up for more widely spaced conductors, but I do know that if you space the conductors more closely, the Q of a loop seems to rise, making it more useful on the high end of the band.