You can fit a 4th-degree rational function that's accurate within ±0.0045, but that's not "simple" and it's not within 1 ppm.
Code:
y = ( 0.014276438 - 0.802004924*x - 15.16719888*x^2 + 66.06068525*x^3 - 36.17009406*x^4 ) / ( -0.017352118 + 0.530402174*x + 47.33893283*x^2 + 22.84085288*x^3 + 63.98324943*x^4 ) + 0.001
There's a better way...