In modeling the pressure filtration of flocculated suspensions using compressional rheology, the filtration diffusivity function D(phi) plays the role of a diffusion coefficient in determining the time scale of the filtration process. Its dependence on volume fraction is an important factor in filtration process design. The volume of filtrate expressed per unit membrane is V = beta t(1/2). The value of beta depends on D(phi) and various relationships between the two that involve data differentiation have been discussed in the literature. These involve an unknown but bounded parameter Here a physical approximation is made that justifies setting this parameter to zero. Further, a new approximate expression for beta(2) in terms of D(phi) that avoids data differentiation is determined Alternatively the hindered settling factor can be recovered fr om an analytic expression involving data differentiation of beta data with applied pressure. The accuracy of each of these approximations is assessed.