the grease+ a little abrasive, gets used .. 'carbon assembly paste'..

generally , if you are conservative on the wrench,
tight enough is just when the post stops slipping down..

I was not happy with the bolt force, I was using, but still getting slipping,
so I added a band clamp around the seatpost itself , sitting on top of the frame's seat post clamp.

Worked OK before on my Brompton, too, though the seatpost band clamp is another QR in this case..


but, I understand, the OP doesn't have a round seatpost..