how to fit the data properly to custom eqaution?

I am trying to fit the data with the custom eqaution, however the line is not starting from below 1, is it because the 3rd term has no x value in the equation? how can i fit this line with data properly, any thoughts on this?

Answer 1

[Torsten]

If you want your model curve start at (0,1), delete the constant term k3/3500*3502.

Your experimental curve looks more like an exponential (exp(k1*Cyclenumber)) than a quadratic in my opinion.