how to find the intersecting curve between the fitted surface (cftool )and a plane ? functions after centering and scaling

I try to work out the function of the intersecting curve of a set of x,y,z points data and a plane, for example z=5. My idea is to fit the data using a polynomial function:z=f(x,y)and solve the equation f(x,y)=5.

The points in XYZ can be fitted into a polynomial function using cftool. However, the cftool ask me to center and scale the data to avoid bad conditioned matrix.

Without the centering and scaling, coefs are:

       p00 =   1.357e+04  
       p10 =      -5.287  
       p01 =      -18.78  
       p20 =    0.002775  
       p11 =    0.002278  
       p02 =    0.006697

With the centering and scaling:

       p00 =       24.36  
       p10 =      -4.337  
       p01 =     -0.4805  
       p20 =      0.1175  
       p11 =     0.09394  
       p02 =      0.2691

I know the centering and scaling is to replace the x by (x-mean(x))/std(x) and y by (y-mean(y))/std(y). However, I don't know how to construct a sym object after the centering and scaling.

Is anyone can help? thanks in advance.