It’s at least partly dependent on your current Progression Level, for a given level.
Let’s looks at my VO2 Max since it is mid-range and 4.3 currently:
Achievable: Ranges from 1.0 to 4.3 (Current PL)
Productive: Ranges from 4.3 to 5.3
Stretch: Ranges from 5.3 to 6.8
Breakthrough: Ranges from 6.8 to 8.8
That may give some frame of reference, but I would be cautious about pushing that too far. It may well have different relative ranges for other progression levels and/or values of PL.