With a matched-flow pump, you do not have to adjust the flow rate manually. The pump dynamically adjusts the flow rate to the incident solar energy. With a high-flow pump (between 40 and 50 litres per hour and per square metre) or a low-flow pump (approximately 10 litres per hour and per square metre), you first manually set the maximum flow rate when using the pump for the first time.
After a constant flow rate has been established, you can now reduce the flow rate as needed. Once you have set the flow rate, you can check from time to time whether the flow rate still meets your requirements.