How do I adjust the corners of my surface fit using a custom equation to better fit my data ?

I am trying to fit a custom equation approximating a hemispheroid to my data, however the corner points seem to stray far from my data as you can see from the attached 3Dplot:

As you can see some of the corners a further from the model than others?

The custom equation i used is: z=a*x^2 + b*x*y + c*y^2 + d*x +e*y+f how can i adjust the parameters of this equation to better fit my data ?

[Star Strider]

The only option I can suggest is to add a ‘weighting’ vector, and ‘weight’ the corners more heavily than the centre.