A dilute acid takes the calcium carbonate crust off regulators and valves, but a bath that is too strong attacks the chrome faster as well. The calculator gives how much phosphoric, citric or acetic acid to weigh into water for the strength you want, from a liquid concentrate or from citric acid powder.
Everything is weighed, water and acid alike, because the strength on an acid’s label is a percentage by weight. Mix by volume and the bath can come out half again as strong as intended. If all you have is a measuring jug, the calculator also gives the volume of liquid acid.