Record Clamps tightness

How tight, when screwing down a record clamp?

Stock clamp with Music Hall MMF5.
Hi, I have a SME 20/2. I use a washer over the spindle (that SME provided) kinda slightly ever so dishes the record down when screwing down the clamp. If I screw it to tight, the record warps up, just right the record touches the platter surface...I can hear improvement when clamped right...