[jaggederest]

Caffeine is only "sparingly" soluble in water (as opposed to "freely" soluble in boiling water).

Chemists have wonderful words to quantify the exact degree of solubility under various conditions: "sparingly soluble" means it generally takes 30-100ml of solvent to dissolve 1g of the material in question.

So, if you're hip to the quantity of caffeine in your beans (by, perhaps, grinding and boiling 100g and then analytically extracting the mass of relatively pure caffeine with something like DCM, followed by recrystallization in water maybe), you can figure out how much caffeine will be present in your cold brew based on the quantity of solvent.