jja, now that I think about it, I believe my problem with getting enough developing agent into a 2-roll tank had more to do with rodinal at 1:100, not HC-110. I think the minimum amount of rodinal concentrate is 8ml per 80 square inches, but I could be wrong on that.
I do recall at least one time though watching thick, syrupy HC-110 ooze out of a 10ml graduate, thinking, "wait, there's a non-trivial amount of that stuff stuck to the sides of this cylinder, huh?"
In the spirit of that, I have this tip to add: a good way to measure out liquid chemistry accurately is to measure your chemical into a graduate; then put maybe 75% your water in a beaker, pour the chemical in, then rinse the graduate into the beaker until the solution is at the proper volume/dilution.
Regarding the minimum suggested amount (3ml) of syrup per roll, I don't think there's anything wrong with Dilution H and I use it myself, but you don't want to be riding too closely to that minimum amount.
Here's an interesting, real-life scenario (as best as I can remember!): 2 reels of 36-exposure film in a stainless tank, which, per prior tests, was found to need only 450ml of solution total to cover both reels. Dilution "H" calls for 7 ml of syrup for this volume, so that gives me 3.5ml per reel, enough to develop both rolls -- barely. But my QFC syringe was off by about 10%, so when I thought I was measuring out 7ml, I was in truth measuring out maybe 6.3ml, so there was 3.15ml of syrup per roll. Now we're cutting it pretty close!
To put that in terms of temperature, a 10% error would get you just over 61 degrees instead of 68. In terms of time, a 10% error at 10 minutes of development is like being off by a full minute. And here I was, sweating how many seconds it took me to pour in my stop bath! Indeed, working with high concentrations magnifies any inaccuracies inherent to yourself and your equipment, so scrupulous lab practice is a must.
I'm surprised to hear that BOTH the picture of Roan and the outdoor scenes were developed in dilution B, since that effectively eliminates the developer concentration as the possible culprit for your flat negatives. The other obvious variable would be agitation technique. 10 seconds per minute is pretty much standard, but it could be that you're just being too gentle with your film. 🙂
Another possibility could be that your tap water is acid or too cold, or there could be residual cross-contamination from stop if you're using it. There are so many variables.
Anyway, I'm glad to see that this thread is garnering a positive response, so if any others have a similar experience or any advice to share, please do!