Global Error for a Fitting Function

I'm trying to find the global error between a data set and a fitting function. My fitting function is a pretty good fit but the error I'm getting using my script is really large. Any advice?

 plot(xC,yC,'.');
hold on;
x=linspace(0,8,400);
m=2;
linfit(xC,yC,m);
y=11.7601.*x.^2-35.7861.*x+120.6944;
plot(x,y)
E=zeros(length(x),1);
for ii=1:400
    E(ii)=(yC(ii)-y(ii)).^2;
end
err=sum(E);
disp(err)

[Torsten]

The usual indicator whether your fit is good or bad is the coefficient of determination:

https://en.wikipedia.org/wiki/Coefficient_of_determination

Best wishes

Torsten.