Dear @g11657 : Which is the use of a clamp?, some could say to tighthen the LP against the platter but in reality this just can't happens exactly in that way due that LP are a ductile build material and suffers " deformations " under some little pressure .
A very good
clamp/mat is the AT vaccum hold down LP that's the best way to really tigth and flattens the LP against the TT platters.
For clamps alone the reflex type as the Sota Reflex or the Basis one are really fine and don't puts so many weigth/mass down there.
At the end what we want with a clamp is try to put at minimum the distortions/resonances between and a ligth weigth clamp can do it just changing the frequency response resonance I'm talking about.
It's not easy to determine in specific the torque or weigth of the clamp because not only depends on the clamp design but on each LP too because does not exist two LPs with the same surface deformations and that's why the the AT I reffered is really good about.
I used this AT and tested with and with out additional clamp and sounds different either way.
Regards and enjoy the MUSIC NOT DISTORTIONS,
R.