Pressure at the end of the hose

The system pressure reading is usually done close to the pump. The actual pressure at the nozzle is going to be lower. As the water is traveling toward the end of the hose, it has to overcome friction, as the flow is not laminar. The reading on a gauge placed next to the pump would reflect this pressure loss. You would notice that this pressure is not always the same. Many factors are responsible for the result, and these factors are not fixed. How much water is flowing, wound or unwound hose, gravity differences, temperature and many more factors are part of the formula. No calculation, as sophisticated the formula might be, can replace the missing parameters. In order not to exceed the system's pressure capacity, it is prudent to leave a safety margin.

Estimating the pressure available at the nozzle

If you do not have an actual reading from your truck you can estimate the pressure loss. The chart below does indicated the pressure loss per 10 meter ( 100 ft) of  rubber hose. Plastic hose is about 10 to 20% more efficient.

Sample calculation
Max. pressure 200 bar. Flow is 70 lpm and 60 meters of 1/2 Inch.  Calculation;  4.0 x 6 = 24 pressure loss or 176 bar at the nozzle.
Max. pressure 2900 PSI. Flow is 18.5 gpm and 200 feet of 1/2 Inch.  Calculation  17.7 x 20 = 354 PSI pressure loss or 2546 PSI at the nozzle.

Step 2
Select the flow on the top of the chart. In this row select pressure you like to have at the nozzle. Do not exceed maximum pressure available at the nozzle. .