The case I was describing is with the BA-1, BA-2, or BA-3 from thorlabs, using a steel (18-8) screw and a SS washer. In this case, you want to go to ~75% before plastic deformation of the aluminum. In this situation, the screw and the base will be elasticly deformed. More tightness will make it plastic and then slowly drift. Less will just give you less stiffness against vibration.

For the case of the 1/4-20 screw with washer in a fork clamp, I expect it can go more, but probably not necessary. To test for drift, we would need an ultra stable Mach-Zender and a long term visibility test, as was done for the stability tests of the Japanese Super Duralumin mounts that Koji has.

Dennis Coyne has a formula to figure out these numbers. I'm going to get to the bottom of this and make it part of this summer's course on Lab Skills.