When to activate Water injection?

[Hemi T04]

I know that swtched - or staged water injection systems are not very smooth in their operation, so i was wondering along the line of the above post- adjusting the voltage to the water pump to controll the flow- in reference to boost pressure using a pressure transducer and an electronic circuit. Say for example the pump turns on at 2 psi- with 9 v dc- then inceases to full voltage at maximium boost to13.8 v. Would this work ok- or would it kill the pump over time. Or will it need a refernce to rpm signal as well to give good water flow control. this could be done off the coil probably.
What do people think? The initial sytem could be pretty easy to setup.