The Scheil rule assumes zero solute diffusion in the solid zone, while the Lever rule assumes infinite solute diffusion in the solid. Back diffusion models a finite solid diffusion at the micro level using a non-dimensional parameter (denoted by ) between 0 (no diffusion, as in the Scheil rule) and 1 (infinite diffusion, as in the Lever Rule). typically depends on the solidification conditions such as the solid diffusivity, the local solidification time, and the secondary dendritic arm spacing. The interface temperature is calculated as follows:

(15–19)

When back diffusion is included with the Scheil rule formulation, the species transport equation is solved with the liquid species mass fraction as the dependent variable:

(15–20)

The total species mass fraction and liquid species mass fraction are related as

(15–21)