What kind of motor are you using? Stepper or servo?
I did not expect this kind of content on THR, it's hella awesome seeing someone mess with microprocessors and homebrew systems.
Hmm. Why not use ADC to measure your charge and make it all one system? You can have incredibly precise charges using a higher resolution. Using a pressure sensor, of course.
Never worked with an Arduino, but I used an MSP-432 for school. You can use your scale to come up with the conversion after reading from the ADC memory register.
The initial rig (the one that rotated the trickler tube to dispense) I was using a stepper motor. The vibrating one is using a small DC vibration motor as per the instructions for the Open Trickler.
I know enough about microcontrollers and electronics just to be dangerous...never had a chance to play with any serious controllers. I've stuck with Arduinos because it's readily available and cheap. It is easy to find examples wiring and code of simple things that I want to do also.
I have not considered electronic scale just yet. Mainly because of cost and space requirements.