Review on Efficient and Versatile: Jebao DP-3 Programmable Auto Dosing Pump with 3 Channels in Sleek Black by Ahmed Morrison

This automatic dispenser does what you want, although as others have said it needs finer calibration. First follow YouTube's instructions to calibrate it and after tweaking it on my system I've adjusted the dosage as needed. Let's say if I needed 30ml I would set it to 30 which would be a dosage of 34. However, if you set it to 27, it would dose to 30, which is what you need. It was difficult to set up at first but I had no problems with it and for the price compared to others it at least gets the job done.