how to avoid the resetting of coefficients in "curve fitting tool"'s "custom equation"'s "fit option" when changing the fitting formula?

I'm trying to hand fit a surface plot to some data, and letting matlab optimize the coefficients based on a starting value and limits. My problem is, that every time i change the formula of the surface, all coefficients resets. I need a way to freeze these coefficients so I don't have to give start values and limits every time I change my formula.