Take the next step, if you have a constant pressure and you are not throttling the flow, then you have a fixed volume based on the pipe geometry and the flow sensor becomes irreverent. your pipe is an orifice that can be calibrated for flow.

your constant flow rate , 6/liters/min is unchanging so the pipe sets the flow, and all you need to do is to know when the pump is running or the solenoid is open.

as soon as you start throttling the flow, you are in a different part of the curve.

When you get into the commercial meters costing 20 times as much, the tolerance on parts and fit, bearings, etc is 20 times closer. but look at the data sheets of those commercial products costing hundreds of dollars. Below 30% of the listed flow rate, the accuracy starts to depart drastically to completely useless values without using drastically different curves for the calibration.

Warning a user of known problems is what we do. Working around them is also what we do.

Now the potential problems are on the table. if the OP does the testing I listed, he will find out if the meter he has departs from one fixed curve in his, and here is the key, application. If he have a constant pressure, and a constant flow, then the single point calibration will suffice.

If he assumes that a contant point calibration is all that is needed, but in the end, finds his appliction has problems, then he knows where to start looking.

Custody transfer, batch processing, assembly line work spend the thousands of dollars for those flow meters for a reason.

1% error can cost hundreds of thousands of dollars in the lift of a process.