A dose is written as a concentration, but nobody can pour a concentration. Getting from the specification to the jug takes up to four steps, and each has its own relation. Work in metric throughout and take water as 1.00 kg/L, so that 1 mg/L = 1 ppm exactly.

Feed rate from a dose: $W = C\,Q$. $C$ is the dose the water must carry in mg/L, $Q$ is the water flow being dosed in m³/h — for a tower that is the makeup, because makeup is what dilutes the programme away — and $W$ is the feed rate the pump must deliver. The units are a gift: mg/L times m³/h lands in grams per hour, because a cubic metre is a thousand litres and a thousand milligrams is a gram, and the two thousands cancel.

The dose a mass achieved: $C = \dfrac{1000\,m}{V}$, with $m$ the mass added in kilograms and $V$ the system volume in cubic metres. Anchor it once: one kilogram in one cubic metre is 1000 mg/L. Everything else scales from that. And $V$ means the whole system — pipework, coils, the boiler shell, every riser and drop — not the tank you can see from the plant room door.

Product versus active: $D_p = \dfrac{100\,D_a}{A}$, where $D_a$ is the active-ingredient dose the specification demands, $A$ is the active strength on the drum label as a percent, and $D_p$ is the product dose as supplied. The shape is common sense: a weaker drum means more of it. A 25 % product needs four times the dose of a pure one, and the check is that $D_p$ can never come out smaller than $D_a$.

The slug for a closed loop: $V_p = \dfrac{C\,V_s}{1000\,\rho_p}$, where $V_s$ is the system volume in m³, $C$ the target dose in mg/L, $\rho_p$ — rho-p — the product density in kg/L off the label, and $V_p$ the litres of product to pour. Dose times volume gives the mass; dividing by the product's own density turns that mass into something you can measure in a jug. A closed loop is dosed once and left; an open tower is dosed continuously, because it is continuously bleeding the last dose away. That difference is the next lesson.