A fertiliser rate 20% out costs money. A pesticide rate 20% out is either a failed application or an illegal residue, and the second one can end a shipment, a contract or a licence. This is the arithmetic to be exact about.

$V = \dfrac{Q}{w\,v}$ — V equals Q over w v. $V$ is the application rate, spray volume per unit area, in L/ha. $Q$ is the output of ONE nozzle — not the boom, and entering the boom's total here is the most expensive calibration error in the trade. $w$ is the nozzle spacing along the boom, typically 0.5 m, and $v$ is the actual measured ground speed, not the speedometer's opinion. Picture it: one nozzle owns a strip $w$ wide, swept at $v$; its litres spread over that strip IS the rate.

Every printed calibration table burns a constant into this: L/ha = 600 × (L/min) ÷ (km/h × m), and GPA = 5940 × GPM ÷ (mph × in). Those constants are nothing but unit conversions in disguise, and mixing the two tables is a tenfold overdose.

The pressure lever. $Q_2 = Q_1\sqrt{\dfrac{p_2}{p_1}}$ — Q-two equals Q-one root p-two over p-one, where $Q_1$ is the catalogue output at the rated pressure $p_1$ and $Q_2$ the output at the pressure the boom actually runs at, $p_2$. Flow through an orifice follows the root of the pressure: four times the pressure is only twice the flow. Which makes pressure a poor lever on rate and an excellent way to wreck a spray quality category — the droplets get finer and finer droplets drift.

The speed lever. $V_2 = V_1\dfrac{v_1}{v_2}$ — V-two equals V-one, v-one over v-two. With the nozzles and pressure untouched, rate and ground speed are inversely proportional: 25% faster applies 20% less, immediately and exactly. Note which speed is on top — the OLD one. That is what makes a faster pass apply less.

The condemnation rule. $D = 100\dfrac{Q_m - Q_r}{Q_r}$, where $Q_m$ is the measured output caught in a jug and $Q_r$ the rated catalogue output at that same pressure. The trade rule is ±10%: past that, the nozzle goes. Note the denominator is the rated flow, because that is the figure the calibration was built on — and note the sign, because over-delivering and under-delivering are different problems with different consequences.