A front tracking method on a fixed control-volume grid, based on assumed dendrite tip kinetics, is applied to discuss the accuracy of a numerical model where the coherency solid fraction is used in the identification of diverse dendritic regions developing within a mushy zone during binary alloy solidification driven by diffusion and thermo-solutal buoyancy forces. It is shown that this critical value of a solid volume fraction is not constant but changes with time and along the border separating the regions. (C) 2012 Elsevier Ltd. All rights reserved.