Record Clamps tightness


How tight, when screwing down a record clamp?

Stock clamp with Music Hall MMF5.
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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...