I see that the class dipy.core.gradients.GradientTable already has attributes "big_delta" and "small_delta". The tau here might therefore be redundant and could be calculated as (big_delta - small_delta / 3) from these values if the gtab is correctly formed. I believe that gtab could have a function for that, but it might need a specific PR. @Garyfallidis what do you think?