A good place to start is with the fully saturated value of 1. Then modify the value to check how sensitive the results are to it. See the figure below, which gives typical values for clays. For undrained loading, generally B-bar is 1 for a degree of saturation of 1. There are exceptions to this, however, including highly overconsolidated clays.
B-bar vs Degree of Saturation
See the following paper for more details:
Skempton, A.W. (1954) “The Pore-Pressure Coefficients A and B”. Geotechnique (4)143-147