It does not look like a polynomial curve to me. I would try with a rational function instead. Here is my attempt. IĀ imported the picture into Mathematica, traced the black curve with the mouse, rescaled the values so that they have the right range, and then I fit the points with a rational curve. The plot seems to confirm that it is a good approximation:
originalPts = {{122.51588254168202`,
204.24489892928017`}, {127.02236509049423`,
221.52865750038615`}, {130.42278577285526`,
239.81157233003358`}, {135.19447646436674`,
261.8608539614255}, {139.27744833569022`,
279.5372850538925}, {123.52326230852417`,
201.8950314323402}, {129.08852155105825`,
233.34995068271752`}, {134.78330104932917`,
259.49659532495923`}, {141.40733698558506`,
281.2148807472459}, {147.6387004004801,
298.1696989863218}, {156.52831287359328`,
308.0934176282546}, {169.37754459351862`,
311.48150304816454`}, {187.2821780413314,
314.0369582526232}, {200.62482025930188`,
314.7030624249842}, {215.92260157577624`,
313.8087558972775}, {234.60024480385977`,
311.19984788886376`}, {252.22322309237188`,
308.04613245552525`}, {274.14504034510213`,
304.00222224863035`}, {297.7362297828851,
297.8798203187203}, {320.3611569953296,
292.55098693983285`}, {341.4092264911051,
282.98087915483245`}, {359.90800841364717`,
276.7104540755089}, {378.6863896184149,
267.1115640114558}, {399.77557665569407`,
258.7133061593126}, {422.3676098349356,
248.06180703276328`}, {445.5969649074855,
234.01811073215362`}, {467.67091706377965`,
220.77003885964177`}, {485.83870277721655`,
205.78680673567118`}, {500.4004801007735,
193.5317234904752}, {535.1591938109781,
166.49282819760185`}, {566.0960320384079,
138.57812927069835`}, {596.705985810883,
109.16469595598278`}, {620.1717667470796,
84.05626923671105}, {641.7481966511782, 58.68674612889038}};
rx[{x_, y_}] = {Rescale[x, {48.84, 642.8069733449}, {60, 240}],
Rescale[y, {56.5, 384.1}, {3, 0}]};
pts = Map[rx, nera];
model = (a + b x + c x^2 + f x^3)/(d + e x);
ft = model /. FindFit[pts, model, {a, b, c, d, e, f}, x]
Plot[ft, {x, 50, 240}, Epilog -> Point[pts]]