Test device and see how much is in one squirt, mine was about 1ml.
If 40ml a day is too much for you (which is lowest i got) , you can maybe try to lower bottle so moving part wont fully press head and there will be less liquid in one pump action.
Alternatively, when buying bottle , look for one that have thinest piston in pump (spring is inside). The thinner piston, the less should be in one squirt
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.
By the way this sounds great and I'm in the process of whipping up one myself. Great idea!