D316 concentrate is behaving exceptionally well in tests. Here's how its density-curve compares with XTOL for TMY-2. D316 was 12 minutes; XTOL was 6.5 min:
The match is outstanding, except that D316's shoulder is a hair better (i.e., higher). Here are full-resolution crops of scans:
XTOL: 12-4-XTOL.jpg D316: 12-14-D316-12min.jpg
The scanner was manually focussed on the centre wedge. D316's grain is a hair finer than XTOL, which we've seen before with this type of developer, and JPEG file-size is 3% smaller, which objectively agrees that it's slightly less grainy. In loupes, grain looks the same to me. So does sharpness.
So D316 is matching (or slightly exceeding) the image-quality of XTOL.
I've also been making concentrates at various temperatures, and made the annoying discovery that propylene glycol evaporates as visible steam as low as 65C. When mixing, turn on the fume-hood over the stove, and don't worry about the constant steam.
The concentrate can be mixed at 65C, but it takes over 45 minutes to dissolve everything. At 90C, you start risking crystallization after the ascorbic acid and Phenidone are added. The constant steam-loss of PG encourages us to keep hot-time to a minimum. I suggest the following procedure (multiply this 1-litre formula by at least 2):
1. Heat 16 ml of propylene glycol to 85C.
2. Add 2.2 g of sodium metaborate 4-mol. I saw a puff of steam.
3. Stir until dissolved (a couple of minutes), keeping at 85C.
4. Add 4.5 g of ascorbic acid. Temperature will drop to about 75C. Keep between 75C and 80C.
5. When ascorbic acid is mostly dissolved, add 0.05 g of Phenidone.
6. Still until all is dissolved (around 10 minutes at 75C-80C).
I'm running more tests, but so far it's looking good.