I want to know the equation of the reconstructed plane in the form of Ax+By+Cz=D inside the mixture cell(having two phases), Here A, B and C are calculated with the help of normal (or Grad(alpha)) while calculation of D necessitate distance from the cell center. Once equation of this plane is found I will find the intersection between this plane and the cell boundary(Faces) to compute the true interfacial area inside the cell (mixture cell). This interfacial area will then be utilized to compute mass transfer between two phases. 

The other half of the UDF I have already written which gives intersection points and compute area using the surface formed by these intersection points. Now its just the value of which is needed to complete the task.

Is there a other way to compute D

Though interfacial area density can also be computed with the help of Magnitude(Grad(alpha) but this is non-zero in the nearby cells (to the interface cells) also. Hence, interface area exists in the cells where there is no interface.This lead to error in the mass transfer rate between two phases at the interface. Therefore area density or area computed using Magnitude(Grad(alpha)  doesnt give correct mass transfer. 

Waiting for your positive response.

Thanks and regards