To add to the advice already ...

To add to the advice already provided, I have found that such pumps are often selected with too high a pumping capacity. This means that they struggle to be sufficiently sensitive to provide the required dose rate often operating within 5 or 10% of their full capacity. I'd recommend designing to use full capacity or near to it as you can and maybe even select an option to have several pumps in parallel if there is a wide range of dosing flows needed. That way you will achieve much better control and the process will be better at meeting the need.