The example script is in the link you provided in the first post, and would still be the recommended method.
If you want to set the visual thresholds manually, I just verified that typing in the value works. Demo below.
I’m working with the standard LuCa file linked in various other places, but it is a float 32 image with an even higher number of display levels than you describe (2,147,483,647). The default max display level for this image and channel, though, is 28.1.
Maxing that out looks like this:
Double clicking on the max value and setting it to something really large:
Results in my red channel essentially going away completely.
The Max display number itself does not change and is not supposed to change.
Double clicking on the 55.0 again show what the max number actually is:
It may not be the behavior that you expect, but if the display isn’t changing, there is something else wrong. In M6 (and in most previous versions) this should work. The main change in the Milestones is the “Keep settings” checkbox that remembers the settings you have applied when you go to a new image with the same number of channels and bit depth.