Indeed, to my recollection, Delta 20 wasn't explicitly stated in the book. I've used it in my own IC training. I've also experimented with trying to keep the sessions moderate, but more constant, in the spirit of KBSF.
I think Delta 20 is more important when you like to push yourself once in a while. If you give yourself a hard session, either intentionally or it was harder than expected, then take it easier in the next session.
In the end, I think either approach could work. Something I like about planning a 20% change in volume from one session to the next is that it helps enure that I'm not always pushing too hard. That said, there are many other variables, chiefly density. Of course, if you're varying weight and reps per set, you should factor that in, too.
The way I like to approach this program is to enter a session with a plan, but remain flexible, and then analyze afterwards. Since I've started experimenting with double KBs in IC, I often find that I stop earlier than I intended to, but while I still feel good. Then, while taking my notes and updating my spreadsheet, I kind of go through a background systems check. I observe how I feel, systematically and specifically. Am I really tired? How do my knees feel? My old surgeries? Things like that.
That said, I initially set up columns in my spreadsheet for 20% up and down, accounting for sets and reps per set (and I'll add weight, when I start to mix in heavier sets). So, with that automation already in place, I like to have that in the back of my mind when I practice a session. These last few weeks, I haven't been very strict on Delta 20.
However, my tactic for using it was this: As I approach the lower bound, I observe myself, like that systems check I mentioned earlier. If I feel great, then I'll commit to proceeding to the upper bound. If things get too weird or uncomfortable in that window, then I'll stop anyway, regardless of D20. There would routinely be days, though, that I would know that I didn't want to commit to 20% more volume, so I'd stop at 20% down. In this manner, my volume would wave, and progress seemed pretty consistent.
I'm about to embark on primarily changing IC variant from session to session. I hypothesize that the change in per-lift NL from session to session will fulfill any needs for variability, even if the total NL remains the same. Still, I'll be observing how it feels. I might go back to D20 on top of changing IC variant to get even more change from session to session. I dunno yet. Will report back.
TLDR: Hmm. I've been writing long posts lately. I'd say, to summarize, that D20 is probably good if you like to push yourself and need to impose a constraint in order to enforce easier sessions. Otherwise, if you want, go by feel, but don't go hard all the time.
