Hi,
I did not calculate the values...
But regarding the accuracy:
With low frequencies you are in an about horizontal line. This means a small error in attenuation causes a big error in frequency.
With higher frequencues you are in an steap area of the curve, then an error in attenuation causes a much smaller error in frequency.
Thus I assume, if you start your calculations from the higher frequencies you get more exact results.
I'd do such test calculations with Excel, because it's just repeating the same formula with different input values.
And you easily can calculate 50 points (or so) and draw a chart to visualize the results.
Klaus