Not much is needed to securely fasten it to the lower receiver. I don't think there is a TDP specification for that screw, so I'll go with a CYA answer and say: tight enough to securely fasten the grip to the lower receiver without damaging the threads of the lower receiver. I know that's not the response you were looking for but like I said, I don't think there is a TDP specification for that screw. Thanks for the inquiry.
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