One thing to consider Steve- When using a dimmer to control lighting, the dimmer controls power, not light output. When you're making those 5% incremental changes, are you measuring the CBCP output of the light? I'm pretty sure that a 5% change at the dimmer will not be an exact correlation to the measured light output and certainly not exact in percieved light output. This will be different from each light manufacturer and each dimmer manufacturer.
Our eyes change and adapt instantaneosly to light input, so percieved light is like beuaty, as always, in the eye of the beholder. We can get as scientific as we want making all these measurements, but the only thing that really matters is what we percieve.
L70 is noticeable if we replace one lamp and stick it next to one that is at 100%. But, if all the lamps are deteriorating at the same rate, a light putting out 70% next to a whole bunch of other lights at 70% is no big deal. Our eyes (and brain) will make up the difference.