You’re welcome. There are many interesting ramifications once you understand that basic concept: low Vf matters, low drop out matters, high voltage batteries matter, etc.
HKJ’s battery comparator suddenly becomes crucial.
So do these graphs from djozz (post #51).
… and here I skim over some more complex issues, but if we assume that the Vf of the LED is actually tied to the maximum drive level in most applications (due to PWM in most drivers)… then we realize that the lower the maximum drive current the driver is physically set up for, the longer the light will stay in regulation [at the same output level!].