Whenever you mix up a solution of ANYTHING, the density changes from 1.0 (water at 20 deg C roughly) to something else that may be lower or greater than 1.0. That is why in exact photographic work, we measure things by weight or use solutions made to a volumetric standard. In other words, you do not add 100 ml of water, you dilute to 100 ml with water, or you dilute to 100 g total with water, using whatever method applies.
Thanks...that makes sense. But just about all the formulas I have read for cyanotypes instruct one to add (for example) 100ml of water, not add water to make 100ml of solution. Or as in Hershel's original formula, "Ammonio-citrate of iron -- 20 parts, water -- 100 parts" (as per an article on unblinkingeye.com).
At least with LF landscape, a bad day of photography can still be a good day of exercise.