An excellent guide to hand sprayer calibration is available from Kansas State University, with the four page "Hand Sprayer Calibration Steps Worksheet" available for download. That guide, however, is in imperial units, so I offer this modification to the procedure.
As a general guideline, many products applied through a handheld or backpack sprayer will work best when applied using a spray volume of 40 to 80 mL per square meter. To find the application rate of a handheld sprayer, follow these four steps.
1. Measure an area nine square meters in area (3 meters by 3 meters) and mark the edges of this area.
2. Using a stopwatch, measure the amount of time it takes to spray this marked area, using the normal spraying speed and technique.
3. Now, using the same sprayer pressure, spray into a bucket or measuring cup for the same amount of time as it took to spray the nine square meters area. Measure the amount of liquid collected, in mL.
4. To find the spray volume in units of mL per square meter, divide the amount of liquid collected (in mL) by 9. To find the spray volume in units of L/ha, divide the amount of liquid collected (in mL) by 0.9.
Once you have the spray volume, you may decide to adjust the spraying technique so that the amount of liquid applied is in a more suitable range, and you can also determine exactly how much pesticide or fertilizer should be added to the spray tank in order to achieve the desired result.