Again, I preface my remarks on mouth calls with the disclaimer that a person's experience and mechanics play a major role in which mouth calls work best for each of us.
Having stated that, I have found that using .004 material as the primary (top) reed and thinner stuff (.003 or less) for the secondary reed(s) seems to strike a nice balance between desired roll-over and rasp. Also, I don't use a tension meter, but I tend to stretch those combinations with .004 material fairly tight to get the sound I like.