When I used a band with my youngest, I just wore it at a comfortable level. I wore it because my ab muscles just ached so I set it tight enough that I had some relief but not so tight that it was cutting in to my skin. I definitely think it takes some trial and error.
I used the band too, but also suffered from a pubic shear.
The pain was unbareable. I still hurt there if I twist certain ways.
My OB used crainal sacral therpay on me along with the band and I did get some relief during the pregnancy. Like Melissa said I just tightened it to comfort
They should measure you for the right size in the office.