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 would 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.