Looking at a curve in Excel isn't going to tell you how it actually "feels" in practice. Instead of meddling with Excel why not just implement the equation I gave and see how it goes? Once you have a feel for how it really responds you can decide whether to leave it as it is or adjust it.
>>>>>>>> speedcurveValue = speedvalueWorked with the quotation to calculate the exponential power curve // added speedcurve value
// Pick one or the other version of the curve
// This is the "negative" curve
speedcurveValue = round(speedvalueWorked * speedvalueWorked / 255.0);
// This is the "positive" curve
// speedcurveValue = round(sqrt(speedvalueWorked * 255.0));