It appears that the way the SafeZ value is currently used in UCCNC, is that SafeZ is in work coordinates.
This has a major disadvantage, in that the SafeZ position will move with varying material thickness, or fixture height.
Mach3 gives you an option to specify Safe Z in Machine Coordinates. This is a better option, imo, as it gives a consistent SafeZ height that won't allow a crash into the upper Z axis limit.
Any thoughts?