Questions about how to use prefocus on an FD telephoto lens

The Canon New FD 500mm f/4.5 L has two rings with focus markings - one is a prefocus feature. It has a thumbscrew that can be loosened and tightened.

If I want to take a picture of the moon some night when the conditions are right, how should I use this prefocus to lock in focus now so I don't have to spend time trying to achieve critical focus on the moon and possibly miss the shot? Does it actually work long-term for critical infinity focus? Depth of field is extremely narrow on this lens and infinity focus is very tricky.

Shooting the moon, simply set the lens on Infinity...no need for focus lock!

Even using the more-precise 20/20 vision acuity standard for DOF calculation (vs. 'manufacturer standard'), 500mm FL focused at 10,000m has DOF at f/5.6 of 3,000m - Infinity!
(Focused closer, at 1000m the same combination has DOF from about 800m-1200m, so much more restrictive.)