Starting by preparing Caffenol black and the Orange developer listed below,
I repared that both have the same pH value and if I mix them then the result would have the same pH value but if I mix them fifty-fifty the final solution will have half the amounts of chemicals per liter and the recipe would be then:
1000 ml water
10 g soluble cheap coffee
23g Sodium Carbonate*
5 g of Vitamine C
What changes then? The developing time, because coffe and Vit. C are superadditive. In fact the developing time drops to 12 or 15 minutes. The first roll I have developed in just 10 minutes but the film is slightly underdeveloped. Next time I will try 15 minutes. To the 1 liter developer I added a pintch of Potassium Metabissulfite to see if it helps to preserve the solution for reuse. I will test this tomorrow.