there's really no "best"......just use something that is thick enough to not bend under the pressure of a sngged lock screw and make your mortice a depth that reveals some of the plates edge. obviously,...the thicker plates will be easier to sink and still leaves some edge exposed. it might be better to think in terms of,..." what is the minimum thickness " rather than "how thick", because plate thickness can vary some from plate to plate,, as long as some exposure is kept, without being detrimental to the gun's architecture. that said,...later guns show no reveal, so a lot depends on what era the build is from. and even then, nothing is set in stone.