So my recipe was my standard high-relief tissue, and it was a small batch.
125 mL Water
0.8g M.Graham Lampblack Watercolor
10mL Isopropyl Alcohol
I pour onto Dura-Lar clear 0.005" film.
The OP had said that 4mL for an 8% gel solution was pretty high contrast. Since I'm using more gelatin, and wanted a lower contrast, I used 10mL for that batch. 6 drops is waaaaaayyy different than 10mL (using the old rule of thumb that 20 drops = 1 mL)
In doing some digging around, I've discovered something. The other manufacturers he mentions (Kiwo, Ulano, Nazdar), all use the same sensitizer, Diphenylamine-4-diazo sulfate CAS # 41432-19-3. So perhaps the 4mL / 100mL (8% gel) is appropriate for these.
Speedball uses something different: Polymethylene-p-Diazo Benzene Dye CAS #71550-45-3.
As far as I can tell, Speedball is the only one that is still available separately (without having to buy and then discard the screen emulsion).
I won't get a chance to try anything else until after the 4th. But if we continue to have a cool summer, I can make some other tests. If it warms up, then it gets too warm for me to pour tissues (no A/C), and it will have to wait 'til fall.