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Chris --
It's difficult to troubleshoot problems that involve a blend of Zone System and BTZS terminology (N-numbers, especially, are imprecise) but here's what you appear to be concerned about: First, the general assumption is that the "normal" subject range is 7 stops (or 2.1 in log terms). Then, the nominal average exposure scale for grade #2 printing paper is 1.05 (equivalent to 3.5 stops). Ignoring such influences as flare and reciprocity effects, the "normal" average gradient is therefore 1.05/2.1 = 0.5. But this is just the way things are supposed to behave; papers vary considerably and even working within the extremes of "grade #2" (ES >0.94 <1.05) the "normal" average gradient can vary from 0.45 to 0.7.
In general, negatives whose density ranges are a bit less than the paper's ES will print easily and well, so your calculated G-bar values of 0.31 to 0.38 aren't outlandish for a paper ES of 0.91 (a soft grade #3 value) although they suggest subjects of fairly high contrast (SBRs 8 to 9.5 stops, approximately). You're asking sensible questions, but I'd suggest forgetting the N-number calculations; they have no fixed BTZS equivalent values, and will simply confuse the issue if you get serious about making the transition from ZS to BTZS.
You're absolutely right about DDX; it's a fine developer but Ilford's dilution recommendations are not realistic (but most other manufacturers' development recommendations are as bad or worse). We've generally found that DDX works well at 1+6 dilution at 70° with constant agitation at around 8 minutes, but if you prefer to work with a #2.5 paper filter you might find higher dilutions — say 1+7 to 1+9 — more suitable for the 8-minute time.
Again, I'll suggest that if you want to make sense of all these numbers it will be helpful to forget zones and N-numbers, and stick to stops and logs. Attempting to blend the two methods will almost certainly be confusing and counterproductive.
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