I think it might also depend on how high users have their display's sharpness and detail controls set too. Having further processing after the Darbee might exaggerate certain side effects for example. I have my X35's sharpness set very low since I've found that I can see the start of ringing pretty much as soon as I go above 0.
Perhaps this means that I can run the Darbee slightly higher than another user that might have the same projector controls set higher? Of course it's also screen size, environment, transfer quality and of course personal taste.
On other thing that I've found is that I get a better result with 2.35:1 by having the Darbee after my Lumagen. Someone who zooms or is CIW will be sending 1920 x 800 pixels into the Darbee for 2.35:1 films. Since my Lumagen does the vertical stretch first, the Darbee gets 1920 x 1080 pixels for all aspect ratios (even though 2.35:1 films are effectively upscaled).
If I did this the other way round, then the 1920 x 800 pixels would pass through the Darbee first, then the Lumagen would then scale this result, so the Darbee effect would be 'stretched'.
So just comparing these various possible configurations shows that the same disc in the same room might give different results and require different Darbee settings.
As it is more of an enhancing device, then of course we're all free to set it as high (or as low) as we like. All I know is that it looks great to my eyes with it set to 45-50%, but I'm sure there are others who would prefer it higher or lower, but they aren't watching it with me.