It is rather difficult to express our ideas in such a way that the others understand and may reproduce what we have done. So, and in order to avoid the killing of a (for me) great developer, I will try to pass all relevant information here in the blog. The recipe must be updated as follows:
500 ml water
20g soluble cheap, not arabic, not decoffeinated
Sodium Carbonate anhydrous q.b. to reach pH of 10,4 to 10,6 (it is 5 g or more per liter)
Water to make 1 liter
First dissolve the coffee in 500ml water and then the alkali, in this case Sodium Carbonate. Stir until all grittiness is dissolved, addd water to make 1 liter and use immediatly. Make 60 minutes semi-stand development at 20ºC. Of course you may reduce the quantities to the half if you don't need 1 liter of developer.
I express here my deep gratitude to Reinhold of the superb blog Caffenol, for having tried the original recipe where I wrote 5 g of Washing Soda as a fixed value. He only got 9,9 of pH value and the Stripe of Polypan F was underdeveloped and gave no details in shadow.
Here an example of what you may reach with Caffenol Solo and Polypan F: