Here are a few notes regarding my experience with the Digital Pre-Infusion unit. Feel free to share yours, too.
Prior to the digital pre-infusion unit I found that I was spending considerably more time dialing in my new roasts. I was trying to get my six-seconds rule-of-thumb pre-infusion time to marry up with my 1-1 ratio of coffee weight to seconds extraction time, strictly by adjusting the grind settings. This often took multiple shots to dial-in and sometimes resulted in a little settling on my part . I have found that adjusting the pre-infusion bar settings to match my roast (light versus dark) speeds this process up and often leaves me with noticeably better results in the cup. (i.e., I can taste the difference)
I did have to adjust the bottom nut a bit when I was exploring the maximum setting of 6.0 (I saw this recommendation by Reiss in another thread); however, so far I have found only one super light roast that benefited from this maximum setting.