A key concept that might be used for applications is the (numerical) derivative of the solid fraction fs with temperature.  A numerical example using a copper zinc alloy at composition Zn 30% in weight is proposed as an  example here using the opposite sign for using both temperature and its derivative in the same graph.
 Kozlov and Schmid-Fetzer have calculated numerically the derivative of the Scheil curve in an open paper https://iopscience.iop.org/article/10.1088/1757-899X/27/1/012001  and applied it to the growth restriction  factor Q in Al-Si-Mg-Cu alloys.