First of all, I'm very green when it comes to anything that isn't mechanical or basic electrical.
So be nice. 
For 1 of my project cars I want to be able to monitor the injector open time/duty cycle, apply a fixed percentage reduction and then output that new duty cycle.
What I want to create is a water injection system for a supercharged or turbocharged (I haven't decided yet) V6, using ethanol compatable (corrosion resistant) fuel injectors, with 1 water injector per cylinder and maintain a fixed (on boost) water/fuel ratio that remains constant as the fuel usage changes.
My (actual) fuel injectors are rated at 440cc/min and apparently 200cc/min ethanol compatable injectors are available. But that has the water(ethanol) injectors flowing 45.5%, (which is atleast 33% more than I would want. Maximum 30% water/fuel ratio) of the fuel injectors and that's before we allow for allow for the higher water injection operating pressure (100psi) that will increase flow even more.
I found http://www.benripley.com/diy/arduino/three-ways-to-read-a-pwm-signal-with-arduino/
Would 1 of these methods of measuring be of any help to get me started?
Any other thoughts or feedback would be great.