Logic circuit shutting off at target voltage
What you are doing is analog state, and you shouldn't use logic gates for this type of problem.

The reason is that, logic chip are not designed to get an analog input, while it will switch at some point, it will be fairly random.

Better suited is an op-amp, which would be used as a comparator  opamp comparator

If you want to make things better, you can also use a Schmitt Trigger that will avoid transiant states and oscillations.