Instead of modifying the doser you could also just dilute your fert mixture. Twice as much water would allow you to keep the 40ml per day but only put in 20ml worth of ferts. Use whatever calculation you need to get the right amount without the hassle of fiddling around with the mechanism.
Yea, im doing that with my two peristaltics which dose ppspro. I wrote a calculator which calculates dosing if we give volume of aqaurium, volume of fert and amount of dosing. I know mix for mine aquarium but people come and beg
By the way this sounds great and I'm in the process of whipping up one myself. Great idea!
Thanks all, this is just a something in return to community