Comment 4 for bug 1482163

Maybe this comment will be helpful:
https://github.com/mixxxdj/mixxx/blob/479c9e15159299667f2f7d16d65a8565f656a376/src/preferences/dialog/dlgprefcrossfader.cpp#L203

So, the xFaderCurve is in the range of 1 to 1000 while 50 % slider results to about 2, which represents a medium rounded fader curve.